Putting a V8 into a 1953 model can be a challenge unless you use an independent front suspension clip. Possibly this is already in your plans if you plan on going to disc brakes. The original 4.56 rear was not made to run 75 with any kind of engine. Chevy 3speeds with a Warner overdrive makes a 4.56 very driveable. Your plans of installing a v8 will require many hours of labor. It could turn out to be such a project that you don't have time to go boating.
I am open to ideas do you have any recommendations on another GM engine 6 cylinder? And the boat engine is next FYI last weekend I blew the solenoids, due to bad seals, which in turned flooded my engine with salt water, and cant even get the thing to turn on to flush it...
