Roughly What kind of speed can I get out of a 27ft Fever with a 500hp 502 with a 24pitch 4 blade prop? Or what would be a good pitch to run?

If talking Merc 500HP or 500EFI and Bravo prop, you will need a 26". Remember the 24" is just about 23" stock. Is your drive ratio 1.5 or 1.36? Speed will be low to mid 70's.

I believe it is 1.5. And The engine was a 502 mag MPI but has been rebuilt milder with a carb.

If you have 500hp then I would say a 26p B1 as well. Do you know at what RPM your engine makes the best/most power? That will determine what RPM range you want to be in at WOT and lead you to what size prop you will need.