I like your boat! I have a 76 Sleek Craft that looks to be from the same mold that I am restoring (slowly). If your biggest problem is a flat cam, you could feasibly pull the motor down, clean all the metal out that the bad cam left behind, then just do rings, bearings and a valve job. Your back on the water in a couple weekends. If you want 400-500 hp, I think you'll need to do a complete motor build with a good plan to start. A pump gas 450 hp motor is very doable, but you're going to need good parts for it to live ie; a decent set of heads that can flow a little, decent valve train with preferably a Hyd roller cam, decent pistons (not stock cast), a cast crank is fine as long as it's no smaller than 10-10 and I would Tufftride it for added peace of mind. A good balance job, and I would go at least 750 cfm on the carb. Get with a good machine shop that knows marine engines, this is probably the most important part of the build, even if you do your own assembly. A good machinist will help pick out the parts, piston size, cam, etc, that will match your motor so everything is working efficiently together. This is just my opinion, so take for what ever it's worth.
