If take .040 off the heads, and later replace pistons with flat tops, will the the valves still clear the heads with a .450 lift cam or would I be upgrading heads at the same time?
Shouldn't be a problem. As your motor sits in the picture you posted, what will make or break the performance of it will be how the rings seal, and how the valves are sealing, and I'll throw in valve guides being in good shape.
