I don‘t believe that it shares the same intake etc as the MPI. If we’re talking the same engine, it has a throttle body injection setup on top of a flat intake. There is also a 7.4 MPI that is similar to the MPI but it has a different t intake and isn’t performance oriented like a 454/502 Magnum MPI (these have forged bottom ends/big rods).

I believe this is correct. Regardless, unless is was a Merc "blue" intake system I'd probably be changing it up anyway. And yes, I believe the crank of the 300/330 horse engines is cast, not forged. I'll be replacing that when I rebuild as well.

The block should be a 4-bolt. If it’s a Gen VI then you have a good foundation for stuff later. It’ll probably run good in your boat now (once you get all the glasswork done).