At 500 miles and the builder using quality valve seal, I doubt its your valve seals. But if you do replace them, you only need to replace the intake valve seals. That will cut the time down in half for you. But I would inspect the valve guides for excessive wear.
Shouldn't he do a leak down test before disassembly just in case it's the rings (or ring seating). If he plans to pull the heads (even though he doesn't have to) to replace the seals which is the only way he can check the valve guides??