I need to explore engine replacement options for my single engine 28'. My old 360 is frozen. A new long block seemed reasonable at first, but I don't want to put a bunch of tired old parts from my old engine on the new long block. The new engine should have the center manifold risers and fresh water cooling. The only new part on my old engine is the alternator. The wiring is a big mess and I think I have some old after market electronic ignition. Is there a source for more complete Chrysler engines? Just figuring out all the parts I need for the long block gives me a headache. Do I need to stick with Chrysler just because of the mounts? There are some nice GM's out there that are still in current production and are sold in more complete drop-in configurations. What's a fair man hour breakdown to:

1. Build up and install a long block,
2. Switch to a center manifold system and,
3. Install fresh water cooling.

Can I buy a clean wire harness to connect to the Marinette leads or will I get a big mess like I have now? Should I ponder diesel while I sit on the dry land? What kind of a deal or contract should I try make with the repair yard? Fixed price or labor hours? The yard owner where my boat is says the old engine is very tired anyway. He says he will look around for a better used engine from a repower or salvage. Says he can test it and give me a 90 day warranty. If the price is right this might be worth a try, but who pulls a good engine?

I bought a used Mercruiser 330 (454 CID) for $2000 a couple of years ago. It has rear dump (log) manifolds. It had recently been rebuilt for resale and the seller was anxious to unload it. It fit nicely in my 28 Express with minimal modifications to the mounting locations. I do not recommend going this large unless you plan on replacing the entire running gear. As you can imagine, the boat planes quickly and cruises effortlessly. The extra weight adds stability in rough weather. I also used the opportunity to replace the old fuel tank with two new aluminum tanks, increasing the fuel capacity from 50 to 150 gallons. This also increases the accessible storage area as the tanks are now alongside the engine rather than behind it. Downside is the extra two inches or so lower at the waterline aft--the exhaust ports are now partially submerged. This is not a big problem as long as the flaps are good and the exhaust hoses are attached securely and intact.

In reply to the engine swap, none are ever a piece of cake. I had my mechanic build me two very identical engines. This is important in a twin-engine environment, that both engines be as closely matched as possible. Essentially, I did away with everything in front of the transmissions. Make certain that you use 4-bolt main blocks. The Chevy 350's have Osco Mercruiser type center rise exhaust manifolds, Edelbrock high rise intake manifolds, Mallory point style distributors. My horsepower is probably around 300hp. As to the wiring, I used the original Chrysler wiring harness, and just hooked it up to the various points. I used a Crusader style bell housing, and I even installed fresh water cooling. I also had to get new engine block motor mounts. I now have more room in the engine compartment as well. I really can't tell you a comparison on the engines. My 318's were shot and should have been replaced long ago. I am still using the same props and everything appears to be okay.

The Chryslers are great engines. The main question is how many hours are on them, were they used in fresh or salt water, and the type of use (cruising or fishing). They have lots of torque. The earlier ones had the dual water pump on the side and none on the engine. The newer engines use a single vane pump with a circulating pump on the engine like automotive types. They also had a problem with "hard blocks" meaning sometimes they had a ring seating problem but you don't see this much anymore as most have been replaced. The only problem with those was oil consumption. Overall a great engine that I wish was in my 37. The very worst problem now is the lack of parts for the engine. In other words, if fresh water or closed cooling and compression is good, go for it.

About the ring seating - I assume the engine doesn't run well if this is present, or not at all. If not, how do you tell if you have a problem?
Oil consumption shouldn't present a problem if watched. Do they make the 440 engine anymore? I was surprised to read parts are hard to get. This could be a major problem I purchased two long blocks from a place in North Carolina. It ran me $3400.00 dollars for both: new pistons, rings, bearings, reconditioned rods and crankshaft, new timing chain and gears, new camshafts, new lifters, push rods, intake and exhaust valves, springs, retainers and rocker arms. I also ordered new raw water pumps, engine water circulating pumps, carburetors, and fuel pumps. They give you two complete engine gaskets sets with the engines.

I tore down my old engines this winter and I was really surprised at how good shape they were in. No ridges in any of the cylinder walls, pistons and bearings showed normal wear. Only problem I found was that both engines suffered from leaking head gaskets and rear main seals. Changing engine oil every 40-50 hours pays off.

What sort of fuel economy do you get with the 350's? I had considered the V6's with a blower, I should be able to get ~300+ out of them with that, but don't the engines sort of act as a keel/counterweight? I don't want to get too little weight down there, especially if I'm gonna be scootin along at 35 mph and a flybridge sticking up there. Also don't want to do anything that would reduce the life of the engines (like adding a blower?) I had figured I would want to go to 1.5:1 ratio transmissions, since the power curve of the 350's is a little higher than the 318's, and especially if I go with a V6. The sedan isn't much different than mine correct? Basically the rear deck above the engines is enclosed right? Did you go with fresh water or raw water cooling? I was leaning towards fresh, since our boating season here in Florida is year-round, the boat will rarely be out of the water and I can't see saltwater sitting in an engine being a good thing. (Good decision.)

You definitely want to go with freshwater cooling. Florida's hot, salty water would eat an engine up. Not to mention having good seawater strainers that can be cleaned while the boat's in use. Grass ingestion is guaranteed if you’re going where the fish are. BTW, are you fishing the Gulf or Atlantic? That would make a difference regarding how much fuel you need to carry. Gulf coasters routinely go 100 miles offshore to find deep water. I don't think you have to go but five or ten miles off the east coast, and getting thru the passes is a much greater challenge. If ballast is your concern, and you're on the west coast, go with larger fuel & water tanks and bait wells. At WOT ballast isn't going to improve handling or stability much because there won't be much hull in the water. On the east coast, I'd prefer to be lighter to minimize draft and maximize speed while transiting the conflicting currents and waves at the inlets.

Thanks for the info. I think I've decided on either a Chevy or a diesel. I could put the same thing back in there but, like you said, I'm trying to improve on them. 20mph is just too slow for me. I want to increase the power and the economy, which pretty much means either a diesel, or get away from carburetors, especially since I will most likely be adding at least another 2000 lbs to the boat in fuel, inverters and batteries. One thought I had, I haven't done any poking around on the boat to see if it would work, but if I switched to diesel, then I would feel fairly safe placing a tank around the cabin. I was thinking of something like a bladder-type(or other custom) tank between the ribs, underneath or on the sides of the cabin. Just trying to find away to move that weight forward some at 6lbs/gal for gasoline, and 7lbs/gal for diesel, just for an extra 100 gals. That's a lot of weight to put that far back. Around 1993 my uncle put a 7kw Kohler diesel generator in the aft bilge, in the space below the hatch, and then took it back out because he had trouble planning the boat after loading it with people and cargo, and it weighed about 600lbs.

I wanted the 15 additional horsepower and the better gas mileage so when I had a new 360 long block put in my 1980 28' single engine sedan I insisted on the newer center risers to replace the "log type" manifold and risers that came with the original engine. It cost me a bunch in additional labor. Water washed back into the cylinders every time the mechanic tried to start it. He got real sick of working on my boat cause he had to pull the plugs and dry things out after each attempt. The risers were too low. (Correct!) How could this be? (The new risers were lower.) On a single engine the engine is mounted low - right down in the "V". Since most Marinettes are twins, the engines are 6 to 8 inches higher providing a margin of safety for center risers. At first we didn't know what to do and I was ready to eat the center risers and go with the higher old log type. The only center riser parts available are the manifold a 1" adapter with two gaskets and the riser, which bolts on top of the adapter. Then the parts guy started playing around and discovered you can stack the 1" adapters like the meat in a sandwich. They put in an extra adapter in and the engine started and ran fine, on land. Originally, when I measured things with a laser level the exhaust ports and hole in back of the riser were at the same level. Now the riser is an inch higher, making maybe 4 or 5 inches up and into the engine. With 5 inch 5/16 allen head bolts we could put in two more 1 inch adapters in on each side and give the riser 2 more inches. That would be close to the hump in the old log style. Bottom line is putting center risers in a single engine Marinette is not a simple bolt on. If I had it to do all over again I would probably stick with the old log type. On the 1" adapter plates stack, this info is nowhere in the specifications. I still don't know how the two adapters are going to work in the water. Do you think I should add the two more or risk being towed in?
What type of elbows do you have on top of your center rise manifolds? I have the Chevy style with the adapter and they’re taller than the original log style.
I had to add a 45 degree elbow to make the exhaust flow downhill correctly, but other than that mine work great.

I know of only 3 types of "elbows" or risers for the Chrysler 360 center risers. The original Chryslers bolted straight on the manifold without an adapter. These are listed in the parts list as "no longer available". The replacements require the 1" adapter(s) and are about 5" longer than the originals and are angled down. The originals have a 90 degree elbow and come off the manifold. The extra length and down angle put the end of the replacement riser at the same level as the exhaust pipe on my single engine. The third type is an all copper "lifetime unit" ($259) which is configured like the shorter original. We did use a 45 degree fiberglass connector to route the exhaust. I made some observations with a laser level on the last unsuccessful test. Working from the exhaust pipe forward the hose after the muffler was pulled up about 1 1/2". Continuing forward, the hose dropped about 3" before attaching to the 45 degree connector and up into the back of the riser (which is at the same level as the exhaust pipe). This created a dip between the muffler and the riser that collected water. The mechanic pulled up the hose to take out most of the dip and put in another 1" adapter and the engine started and ran fine. I would feel better with at least a 3" drop between the end of the riser and the exhaust pipe. They should be connected with a straight run of hose/muffler. For the short original risers, I'd want at least a 6" drop. Same for the similar copper risers. If I was replacing originals I would probably use the copper style.

I am so happy with my new 1973 Express 32 FB. As a first time owner, it has brought a new dimension of enjoyment to my wife, 5 kids, and I. I bought it knowing the Chrysler 318's had about 3000 hours on them. (Wow!) I was going to try to baby another year or two from them but it looks like that is not going to happen. Recently the port engine has been hard starting -(have to use starting fluid), and I checked pressures on one side at 60, 60, 60, and 40-70. (Normal is around 130 to 140 pounds.) I'm assuming that cylinder has some mighty worn valves. I can only get about 2,400 rpm out of the engines and the port engine runs rough. I'm looking for ideas on repowering. My initial thought was to order a factory rebuilt for about $1750 per side and have my local marina install them. When I talked to my service man, he said if you do that you'll still have engines that haven't been built for over 20 years, and the transmissions and wiring will be 32 years old, and I wouldn't have electronic ignition. (Bull!) He recommended buying new GM 350 or 454's and have all new from ignition on down. But that would cost me about $6,000 per engine. I did talk to one local Marinette owner that went with 454's and he is very happy because he has the power and speed when he needs it but he usually powers back and gets much better economy that with the 318's. (Again, bigger motors—properly propped—get better gas mileage.)

Another fan of "more power"! On the Chrysler mills...You can still get all the parts you need for these engines, and they are great engines. Rebuilt marine engines from reputable suppliers use solid, tested blocks that have not been used in raw water service. The rest of the parts--cranks, rods, etc.--should run forever; just check 'em, grind 'em and install 'em. Talk about tough? We've heard from Chrysler engine owners that have run them for well over 3,000 hours--one has gone 30 YEARS and they still run. Yours is a good example: 3,000 hours and it's still going. So don't dump the Chryslers yet.
Other points to ponder: You can spend many $$$ adapting different engines to your boat, where the present ones will go right back in. Ditto the transmissions: rebuild them while the engines are out and you'll be fine (and they last a long time if taken care of). On new electronic distributors, you can buy replacements reasonably priced, and re-wiring for them is a HELL of a lot simpler than setting up a Mercruiser!

I also have repowered a 318 with a 454, but I disagree with the notion that it was the right thing to do. First, with the same running gear, you will not be able to use the extra power available in the 454. It will accelerate faster, and NEVER bog down, but the top end will be virtually identical. Second, the added weight lowers the waterline. This is a good thing if you're looking to improve stability, but it puts the exhaust thru-hulls below the waterline (one more thing to worry about while it is docked). If you want to go first class, repower with either Crusader 350s or Mercruiser 5.7 liter Horizon engines. Expensive but trouble free. If you are going to run it a lot, consider the Yanmar 200 diesel. The diesel option doesn't make sense for pleasure boats unless you are doing long distance cruising or fishing on a regular basis. Either way, the fuel tanks are probably corroded inside and should be replaced with new welded aluminum tanks.

I may install a rebuilt Chrysler 360 CCW long block this winter to replace my 318.. I can only get 3,600 rpms on my 28 footer with flybridge and a single 318 engine with a 16 x 15 RH prop, and I am not able to get fully up on plane: max speed is 16 - 17 knots. I have done everything I can on the engine and need to decide if I am going to go for a rebuilt engine or try a 16 X 12 prop and be satisfied with the limited speed increase. This deals with my Chrysler 318 (240 hp) engines.

Port engine has to be nursed at idle so it can be put into gear without stalling. If I goose the throttle to a higher idle it'll run just fine, but then it's reving too fast to shift without damaging the trans. It will stall even more readily if I throw her into reverse, and ain't that fun while docking with the wind blowing.

I am restoring a single engine. On long blocks, you will need to re-use your pans and covers, carbs, starters, alternators, distributors, ignition and intake manifolds and cooling system to make it pay. Switching from 318 to 360 costs even more. Some new GM crate engines have all this stuff attached and also use modern fuel injection and ignition systems, but you have to catch them on sale. It is very easy to rack up the cost of a crate engine if you have to replace too many bolt-ons. That’s what happened to me. Also the new engines use efficient "center dump" risers. I put new center dumps on my Chrysler rebuild. Had some problems routing the exhaust. Also service the trannys and replace the transmission cooler. New trans are scary high priced.

MISCELLANEOUS CHRYSLER INFO:

The bolt pattern only allows the flywheels to go on one way.

Number one cylinder on a Mopar is ALWAYS the left front, regardless of rotation.

The distributor can only go in two ways, so if the oil pump gears are lined correctly, the timing will be close, or off 180 degrees.

Use only a marine circulating pump (front of engine) or the impellers won't last.

Watch the exhaust pipes for the water coming out. Make this a habit every time you fire the motor up.

The raw water pump sends the same amount out the exhaust, whether it circulated through the engine or not.


MISCELLANEOUS INFORMATION:

Buy a non-contact, laser pointed thermometer and use it to check the engines and exhaust all the way to the stern. (No inboarder should leave the dock without one.) Any hot spots mean trouble.

A raw water cooled motor can last 20 to 25 years in fresh water, and the engine will need rebuilding by then anyhow.

The drive belts are oddballs. You'll probably have to do a bit of experimentation to find the right sizes. Take the old ones off before they shred themselves and get them matched up at NAPA or where ever.

My engines are carbureted. The EFI motor was 330 hp per engine and cost an extra $1000. I've got too much power already. It's not the engines that get you fuel mileage. Sure they should be tuned up and running correctly but it's the propeller that’s responsible for the losses. On inboards, the propellers drive at a fairly steep angle. The struts also cause a lot of turbulence leading up to the props.. With 1:1 ratio transmission it's even worse. Outdrives are much more efficient. The new Volvo drives with the props on the front would probably pick up 35% in efficiency over my setup.

A V-6 setup with the proper reduction gear would definitely improve on my engine setup. Not because the engines are smaller but because there is a significant weight savings and efficiency gain from the reduction trans.

REPOWER COSTS TO CONSIDER:

The base cost of the engines is just the beginning. Labor is expensive. The new engine has brackets for the trans and throttle to hook up that are different, as are the mounts. Need to fabricate new brackets or buy new cables. May need new props.

To repower a 32 with diesels is 2 to 3 times the cost of gas. There is not enough air intake for the diesels so custom vents will be a major item. Also a big jump in torque may require a drive train upgrade. You will never make the cost back. But you never get your money out of a boat anyway so it is a matter of preference. I think I figured out the break even at 600 hours / year for 5 years. Time to repower again. That doesn't include the higher cost of diesel mechanics/parts. Also I'm not convinced a 3600 rpm, high-speed diesel will outlast a properly cared for gas engine.

This is an interesting thread for me to read as it brought back some of the things I went through when I had to make this decision a while back. I guess like everyone else I considered GM with EFI and diesel for repower, I gave the Yanmars some very serious thought for about 8 minutes; they make no sense at all. I put in the Propower Gold factory remanufactured small block Chrysler with a 2 year warranty, new manifolds elbows and risers and a new circulating pump. Total cost was about 2600.00 each engine. The best part of this deal is that everything bolts right up and you don't have to reinvent the wheel. While I was at it I put in a Sendur closed cooling system I found used for $175. I had arranged for a mechanic to help me with the swap but he had one issue after another that kept him from getting here so I started slowly taking things apart one at a time. The next thing I knew I was lifting the old engine out of the boat. I did it right at the dock with a davit I bolted to a piling, a come-along rigged from an A-Frame I built in the cabin and some wooden cradles I built with casters to roll the blocks in and out thru the sliding doors. The worst part of the job was cleaning and painting all the cross-over parts. You have to pay attention, take pictures and make maps of things like the distributor wiring pattern etc. It helps a lot if you know a mechanic that can answer questions from time to time.

I think the 32 FB sedan is probably one of the nicer boats to do this job on. (The Express is a LOT easier!) I think anyone that has changed manifolds, pumps, sparkplugs etc could do this job with a little help from a pro. The key is to take plenty of time and think everything through twice. The $2600.00 looks real good when you price a Honda 225 hp outboard, it's not much more than the sales tax on it.

Thanks for clarifying the old versus new Chrysler engines. I did hear the Chrysler 318 was a good engine but was not sure how long they would run without needing to be overhauled. Are 318 parts still available in the auto stores or do you have to upgrade to the 360? (Yes, and no.) Is the 318 the same block just bored out, or are they totally different from the 360 block? (Longer stroke and larger bore.) I guess if taken care of they should go 2,000 hours or so.

PLUSES FOR CHRYSLER REPOWERS:

Rebuilt long blocks are cheap and as available as Chevies! The Chrysler Small Block is a non-interference design engine, meaning that the valves are not destroyed if the timing chain should break. (There is no timing belt.) Stories abound of phenomenal engine life on 318s and 360s. Their valves use 3/8 inch diameter stems (compared to 11/32 on all other small blocks).

I just purchased a 1983 28' Sport Fisherman. The previous owner says the engines are Chevrolet 350's. He has no information concerning the engines so I don't really know what is in the boat. How can I identify the engine make and size? Both engines are rusty and look pretty bad. I am thinking of having rebuilt engines installed so I want to have the same size and shape installed to minimize the fitting, etc. Can I get electronic ignition on a rebuilt Chevrolet engine? (Heck yes! The most common type out there.)
I don't think the 350's are original for, given that the boat is a 1983, it probably had Chrysler 318's originally.

Answer: The Chevy 350's are probably newer than that and they may already have electronic ignition. First things first, examine the block and heads to find an engine serial # and heads serial #. You may even find "GM" cast into the block and heads. There are a lot of aftermarket valve cover manufacturers so the engine manufacturer may not show up there
I was sitting here, trying to think of an obvious spotting feature that distinguishes a Chevy from a Chrysler. Very difficult. The best thing I came up with is the valve covers: Chevy uses either 4 bolts around the outside to hold them down, or two bolts through the center of the cover. Chryslers have five bolts around the outside. Also, the distributor on Chryslers is offset to the left of the centerline; on Chevies it's the opposite
.
Seems like there is always something with these Chrysler engines. I noticed a small amount of carbon from my port engine. Now I am more concerned.
Seems like anything over 900 hrs on these engines and bad things start to occur. (Not really.) I am not ready to pull heads, but I would not be surprised to be doing it soon. I will personally run mine until they crap out since short blocks at least will be the minimum rebuild required. I do know that my port engine will only do around 3,000 rpms @ WOT and the stbd around 3,400 rpm (with 14 x 9 props) so age is taking its toll. I wish someone really had a complete solution for replacement engines. All our threads on repower run in endless circles. I believe the 350 ci blocks are the same. The only differences other than the alternator, starter, fuel pump, carb, flame arrestor, exhaust manifold are:

1. The marine intake manifold is sometimes angled to compensate for the angle of the shaft and the angle of the boat as it tries to plane. The carb is also sometimes angled by use of a wedge plate under it.
2. The oil pan is different on marine engines- deeper, more volume, sump and pickup location and it is sometimes aluminum vs. stamped steel.
3. The marine cam is not a typical street cam. It produces more torque like an RV cam.
4. The marine circulating water pump has brass or bronze vanes instead of iron.
5. The thermostat opens at a lower temp (especially on raw water cooled engines) and is made of stainless instead of brass.
6. The marine bell housing which mates the engine to the trans, sometimes has the starter mounted near the top of the engine instead of near the bottom as on a street engine. I think this is to keep it high and dry and serviceable.
7. Sometimes the vibration damper at the front of the engine is mechanical instead of fluid.
8.The distributor is sealed against moisture and fuel vapors on a marine engine.
9. The oil pump used on a marine engine is NOT a high volume pump
10. Freeze plugs are brass, not steel.
That's all the possible differences I could come up with.

I read that GM makes two different lines of its 454 ci block. One is "regular" and the other is heavy duty truck/industrial. Crusader supposedly uses the heavy duty block for its marine engines.

When I was playing with exhaust manifolds, the only gasket I found locally that would work was for an ancient Mopar 273 cube motor. The standard drag racing header gaskets (for a 318/360) were off by a mile. Ditto for any other 318/ 360, normal car/ truck gaskets. The bottom line is, trial match the gaskets to the head and header, being damn sure they cover at least 1/4 of metal all around. If not, the sucker will blow out on you.

I didn't see it mentioned but generally marine engines will (sometimes) have a heavy duty valve train from timing chain through rockers, pushrods, springs and lifters. Also forged pistons. A marine engine is usually designed to run for extended periods at 3,000 or 3,500 rpm. Don't try that with an automotive engine.

My 318 has four bolt main bearings as well. When my auto mechanic friend helped diagnose our cracked piston, he was surprised to see that our small block Chrysler 318 had 4 bolt main bearings instead of standard 2 bolt mains. He said that 4 bolt mains were an option on heavy duty truck engines.

The 454 fits into my 28 Express just fine. It is only slightly longer than the 318. And it came with a 1:1 tranny, possibly because it is a 1978 model.
The prop size does not determine the amount of torque in the shaft. It comes from the engine, and can be multiplied by the tranny (a 1.5:1 produces 50% more torque than a 1:1). My 330 hp engine with a 1:1 drive is borderline for safety with a factor of 1.96 (the shaft is 96% stronger than the maximum stress that the engine could produce). A factor of 2 is normal design. You could drive a higher power engine thru that shaft, but my original point that the benefits are slim holds. I don't notice any degradation in fuel economy due to the extra weight because it jumps right up on plane with little throttle. It does sit about two inches lower at rest, which means that the exhaust thru hulls are now partially below water. I prefer the extra weight (I also tripled fuel capacity to 150 gal) because it stabilizes the boat somewhat in crossing waves. You should adjust trim with the tabs rather than using ballast forward. Mine quit years ago, so I fastened them in a 5° down angle, just enough to keep the bow down at cruising speed. I think you could use these engines if they fit, and the extra power is nice to have when turning, accelerating, or late in season when the hull is fouled. Your mechanic should be responsible for determining if they will fit, and how he will get them mounted, piped and wired.

The previous owner of mine said he put "360 heads" on the 85 (360) engines since they came with 318 heads--it was a 260 hp. engine package. From what I've discovered the "big valve" heads weren't much different, and Chrysler used them helter-skelter on both 318s and 360s. The exhaust port size and shape resembles a 273 cube engine. Try using an automotive header gasket for a 318 or 360 and you'll be amazed at how much larger the exhaust ports are.

Just had a long conversation with my marine mechanic and retired Chrysler mechanic (who used to teach classes on Chrysler engines). Said it is a lot of work to change from 318 to 360 for so many things are different. According to him, the 318 and 360 are not the same block. The 360 has different size crank bearings, so you could bore a 318 to a 360 but the crank won’t fit due to the bearing size. The flywheels and damper are different as well (internal vs. external balanced).

I have not seen a Marinette that uses the transmission to reverse the engine rotation. On mine, a 1985 39' Sedan Bridge with 454 cid Crusaders, the port engine is the reverse rotation and the starboard is the standard rotation. I could have this part wrong because I've been down there messing with both engines but I'm fairly sure that standard rotation is counter-clockwise (when looking toward the front of the engine from the transmission end). If your engine ID/name plate is still on the engine, you should be able to decode the model number to tell for sure. People who know about those things can tell by looking at the firing order. I've got the Crusader manual so if you need the firing order for both, let me know. I assume (but am not certain) that the firing order for all 8-cylinder engines is the same unless it is reverse rotating.

Those newer 4.3 V-6s have lots of power--certainly more than the Buick 215s--and are shorter as well (for improved front access). They are a bit wider, though. Going to EFI requires a pair of fuel tank electric pumps and a return circuit (which you don't now have). A four barrel Vortec V-6 makes 200 hp which is enough to get the job done nicely; and it’s much simpler to hook up.
The trannies are first cooled by the raw water coming in, even before it gets to the motors. I've got 318's in my boat which ostensibly produce 225 or 240hp, depending on carburetion (and exhaust system design). I like the idea of V-6's for the obvious reason that space is at a premium in any inboard configuration but I don't think I'd want to sacrifice that many ponies. Do you know of any V6 mills that push out approx 225hp? Not knowing the routing of the fuel lines back to the tank, although it sounds as though it might be more complicated, I think I'd like to try the EFI route as the objective is to get more power/ efficiency/ mileage to the precious gallon of gas. Do you think that the transmission would be sufficiently cooled using an in-radiator trans cooler or do you think that it might be necessary to rig an auxiliary pump to circulate raw water in a water jacket to ensure that the trans fluid remains cool enough?

The same oil cooler is used for the 275 hp. Chrysler 360s, so don't worry about capacity. The standard water routing--first through the cooler--is all you'll ever need. The 4.3 V6 is about 200 hp. with a 4 barrel; not sure of what an EFI would make (though not much more). At cruising speed the carb motor makes the same torque, and that's what counts. The way a Marinette jumps out of the water, two V-6s would do just fine. Also on carbs vs. EFI: You'll need an electric pump in the tank, or right next to it, and a return line. That's a...