Proper gasoline marine engine tuning will include varifying the advance curve. Adjustments are made accordingly.

Ignition Timing and Progressive Ignition Advance 101 was perhaps not taken during training!
Neither was anything studied re; cruiser type Gasoline Marine Engine Detonation potentials.

Without knowing your exact engine build (piston type/cylinder head chamber size/camshaft profile) I'll not make a suggestion. Too risky!

Ignition advance is based on crankshaft degrees beginning at TDC (top dead center) of any given cylinder.
However, we set/check timing on one cylinder only...... seems to always be # 1 (all others follow suit!)

BASE advance is just that! BASE!:
It is the spark lead that the engine fires up on, and idles on. Nothing more than that!
It is consistant at/in this low RPM range... it does not change once set, and the distributor is locked down.
(this would be for a mechanically advancing type distributor system)
The full electronic types do similar.... but may change BASE some during warm up.

Progressive Ignition Advane and Curve:
This is the spark lead in addition to BASE that occurs during and thoughout the RPM range until the limit is reached.
This is progressive and reaches a limit as to when it stops advancing and at an RPM in which this occurs.
This is the critical part of the Ignition system.
Since your engine is not OEM, there are no OEM specs that I can refer you to. Hence the need to know about the components used in this engine!

Maybe your engine builder can help you with this.
If so, he will/shoud be able to provide you with an advance curve.... but this must be suitable for a marine cruiser application..... NOT AUTO!
You distributor should be capable of giving you this curve/limit.
If not, it will need to be calibrated accordingly.

Don't let anyone foo foo you that the curve is not important..... it is particularly important that the cuve not be too agreesive for the 383 if dished pistons have been used, IMO.

Too little, and we leave performance on the table (no real foul here).
Too much, and we risk detonation (big foul).
It's that simple!

All else being good, of course!

Here are some examples ONLY.... Do Not use these!
NOTE: most all marine ignition advance curves DO NOT include BASE advance.
BASE must be added to TA when you calculate this.
Note the straight line curve.
Note the RPM at which it is Full IN!
Note that the Full In advance is approximately 22+ degrees.
Add a BASE of 8 degrees, and the TA (total advance) becomes 30 degrees.
Note that this 30 degrees occurs at about 3,300 RPM... pretty much above of the rpm for worst case detonation.... again, depending on the combustion chamber!


Just for fun, here is one that's completely wrong for marine use. (I don't know what engine this is for.... I just grabbed it from Google Images)

Note the very steep and agressive curve!
Note how high (in degrees) the davance is taken to the limit.
Note how early this reaches the upper level of advance.
Note that the curve begins at a low RPM (a good marine curve won't begin until around 1k rpm as to keep low speed RPM consistant).
Yours or my engine could never use a curve like this. We'd burn the tops right out of our pistons, and likely ruin a bunch of valves in the process, if we did use this.