The single largest differences between a marine engine and the car are machining clearances, cam grind depending on what exhaust and materials used in head gaskets and freeze plugs.
Brass plugs and Marine head gaskets are a pretty big deal if the boat spends lot of time on the water, to control electrolysis.
A machine shop familiar with the added clearances needed on a marine engine should be able to get you going on the right direction.
Depending on what you intend to do to the engine, the clearances will be somewhat of a moving target.
RPM, HP, how long it will run hard, exhaust system, fuel used, compression ratio all pretty much come into play in a small part.
I like forged internals for an all out performance engine. I would not turn cast pistons past 4,800rpm. Forged crank is optional to an extent, I use 500 horse as a guidline. Four bolt block after you touch 600 plus.
JMO based on my experience and the stories of others.
The reason for the added clearances is the extreme temperature differences seen with open cooling, these engines rarely run warmer than 140°F with very cold entering water.
Good luck, sorry for your troubles.
GT
