with boring 502's into what cam do the go with and would you still be able to use stock exhaust

To use any decent cam that you would see a noticeable power difference with, you would need to upgrade the exhaust.

Not sure how married you are to EFI, but converting your engines to carb opens up some options.

Your engines with a nice cam upgrade, some roller rockers, single plane intake, and a 800 Holley, should make 525hp all day. some head work, and even more.

A buddy of mine had some 509's, Merlin iron rect port heads, crane 741 cams, Dart single plane intake, Holley 800 carb, CMI headers, made about 590HP at 5500RPM.

If it were me, your bottom ends should be ok with the hours on them if they were maintained. I'd pull both engines, clean them up, swap the cams, intakes, add carbs, , sell the EFI stuff, and enjoy some nice power gains. Simple stuff, that flat out works. Your stock EFI system will limit your power. Upgrading to a better EFI system, isnt cheap.

540's are nice, but most guy's think the extra 38 cubic inches are gonna turn their boat into a monster. Not sure the extra several thousand is worth the extra 50hp unless your building from scratch.

A properly spec'd 502ci is no slouch.