It would probably make more sense to put in a used good condition GM 350 like a Goodrich replacement out of a truck or abandoned hotrod project and swapping over the essentials from the old engine rather then trying to rebuild the current one. Even better would be if you could find an entire engine and outdrive combination from a wrecked boat, that combo are still used in some more modern vessels but finding a good condition setup from a totaled boat will likely prove challenging. What kind of clearance and room are you working with in this boat of yours?