Are they clearenced for a boat or a car? There is a difference there are threads about people putting car engines into a boat and them blowing up. The exhaust valve clearence and main and rod bearing clearence are different. GM has marine engines and auto engines. be sure.

I beg to differ with that, I am putting well over 300 hp threw my alpha and it is holding up fine, the fluid looks perfect. It is all how you drive the boat, no hole shots and pull the stick back when ya leave the water, it will be fine, I have done uncountable 3500- to redline shots...Alphas are just underated..
John

clearance is clearance, All bearing clearance variation really does is oil pressure, most race engines are "loose" clearance so it will not build too much oil pressure and wipe out pumps.

A 350 car engine and boat engine is clearanced the same.

I tore down my 88 merc 350CI and found nothing different besides the merc had a mercruiser tag rivited to it. same pistons crank and rods as the 88 Roller c am 350 I installed from my friends Camaro. I looked up all part numbers to be sure, both had the same parts, and same balance.

Intake and Exhaust valve clearence has to do with the cam specs and heads used, I ran a decent size cam in my old boat with edelbrock heads and had TONS of clearenc intake and exhast, it was a ZZ9 GM performance cam.

The 92 Merc 454 mag I just had rebuilt also had all the same internals as a 454 HO crate motor. Part #s the same. I think the main differences on a marine engine are gaskets and external parts like oil pan, water pump, fuel pump, carb, and distributer and thats to be Coast Guard approved.

I think Alphas are under rated and can handle more than 300 hp. I know Sea Ray built lots of Pachangas w/ 330 hp 454s and Alpha drives.

I just built a 89 5.7 merc with vortec heads w/ bigger valves and lunati voodoo cam (don't remember the specs off the top of my head). Didn't dyno but it picked up 6 mph in a 19-20' chapparal villain over the 270hp.

When We Build Small Block 383's We Run The Same Clearence As Street Engines Except For Rings We Run The Same Gap As Nitrous. Havent Run Into Any Problems. Also We Run Dart Heads And They Advertize For Marine Or Street Use.
Hope This Helps.