If the boat was raw water cooled, there's a good chance that the block is thin from years of rusting in the water jackets.

Find a good replacement block that had antifreeze in it.

Also, you really need to find a shop that builds marine stuff. There are LOTS of subtle differences that will bite you. (Carb setup, piston to wall clearance, valve guide clearance, oiling, cam selection etc........) don't think you can just drop in an automotive engine and be good to go.