318 marine engine.

Most single engine boats had reverse rotation engines. If it came off a dual engine boat, it could be a clockwise rotation engine. I would get it either way. A crank kit is all it will take to remedy that....and of course whatever camshaft you want. Some of them had unusual gear drives for the camshaft, but once removed, the blocks are the same as far as bolt on accessories. I have never heard anything about them being thicker, but that's certainly plausible, since marine engines and parts are more heavy duty than their automotive counterparts.