Spinning the same main after installing a new crank, bearings, and main bearing caps? Can be the following:
Bent crank (just because it's new doesn't mean it's straight)
Crank journal size is wrong (just because it's new doesn't mean the journals are correct)
Bearing size is wrong
Oil feed to that main is restricted somehow
When the main caps are replaced the block MUST be line-honed
Stepping up to a V8 in place of the V6 can probably be done but it will definitely be more costly. In addition to the engine, pan to carb, you'll need new exhaust manifolds and risers at the very least, and you might have issues using the accessories from the V6 (power steering pump, alt, etc) because the V8 is longer than the V6 so the hoses and wiring may not be long enough. And of course, you'd have to find out if the boat was designed to accommodate a V8.