So it should all swap pretty easy if I buy heads and cams.

The 525EFI cam is a really good unit.

What he said * REmember the 525 is a really good motor and pretty damn durable, plus it usually made well over 525hp. avoid the 525 headers though,Im sure there is something comparable to the heads out there.

We use the 525: system with great results. Dustin Whipple does our tuning. You should be able to use your stock heads, the intake is the limiting factor.

OP has a carb 500 not 500EFI, does that make a difference to do a " bolt on" 525 EFI conversion?

As said before. Strongly recommend checking with Mark at precision or Eddie young. Intake and injectors are a huge limiting factor to these engines. Save yourself time and money and listen to what they say and what packages they have made work.

The 525 head is made for Merc by Edelbrock, really nothing special. The head off a hp500 carb is really pretty close. If he is going to keep it close to stock they should work ok. He will have to make a few spacers to get his belts to align because of the crank target wheel. Unless he has the newer version with it under the timing cover.

Stock 525EFI cam will work well.
525EFI's are 525hp at the prop and dyno in the 560-570hp.

I agree with talking to Young Perf or Precision Marine before you buy parts. They may have a parts list for a similar build and have a PCM tune ready.