Would the HP 500 Heads(with the bigger valves) and cam do any good in a 502MAG? The MAG already has upgraded exhaust if that would be an issue.

I think you will see little performance change as far as mph, should also remap the ecm if its MPI

I will be selling a complete HP500 top end soon should you decide to go that route . Stock carb is a Stage V by Nickerson .

need to do heads(with new valve springs/rockers) cam, intake/carb, exhaust to make work correctly, heads alone on a EFI not worth it

The bottom ends on the carbed 500hp and carbed 502mag are the same.
The 500EFI has better rods.

if carbed ,yes
if mpi,,then yes after remap.