If the engines froze to the point of pushing out the freeze out plugs, then I would say there is a 90% chance that you will find the block is cracked on top of the engine in the valley under the intake. Assuming they are raw water cooled, the salt water will have corroded the head bolts, water pump bolts, and manifold bolts. In salt water repair, the standard practice is to double the flat labor rate for repairs requiring disassembly.

I would just buy new long blocks and be ready to replace the exhaust manifolds too. In salt water with raw water cooling, the life expectancy for a cast iron v-8 is about 5 or 6 years. I have never seen a mechanic try to rebuild these v-8 engines. They just replace them and send the cores into the reman shop.

I left my single 28' with a marina to be winterized and shrink wrapped. They marked the hull as winterized and shrink wrapped it locking me out so I couldn't check the work. That spring I moved the boat 900 miles away and launched it. The engine was frozen and the "freeze plugs" were laying in the bilge. The engine would not turn. The big lesson here is if you have a yard pull your boat you must have them put it back in. That way they are liable.

A further series of screw ups followed.

The new yard guy ordered and installed the wrong rotation engine.
I let him keep it. He had to find another prop and starter.
I had him junk the old log manifolds and install center dumps. They were too low and water washed back into the engine.
We couldn't get the ignition squared away until we installed an electronic system.
The engine still stalls out at about 2400-2700 rpm and you have to back off. I need to check the fuel pressure.

It is true that Chrysler long blocks are fairly cheap. Buy from a re-builder not a broker. The big problem with all long blocks is all the old crap is bolted right back on the engine. You have to seriously consider buying current production crate engines (with EFI to keep it simple). They do come on sale periodically.

On a FEW occasions I have sucessfully installed new core plugs ( thanks Jeff ) & the motors lasted years. Use new plugs & put a gasket sealer around them when you install them. Start motors on a hose don't launch because you may have to pull it right back out. don't reve the motor just let it idle so you don't do major damage if you have water in the cylinders.Watch the oil if you start to get a milk shake your done. If it runs let it run long enough to build up plenty of heat in the block, sometimes cracks do not open until hot.
If you get any water in the motor just replace them, freezing & SALTWATER have killeed them.
I prefer to use long blocks as opposed to short blocks because builders will usually give you a better warranty on long blocks.
As yooper said NEW stuff with EFI is the ultimate way to go but a little more expense & a little more work. New motor mount location & adapting wiring harnes's
Dave