Building a 340 engine

From what I know (or think I know!), neither head is going to get you to 500 HP as described. And certainly not with that cam. The cam is good for low compression engines on the street but is unecessarily limited for the engine parts you have, and the piston options that you will likely be able to use.

We need to be sure on one piece of data on your block to look into pistons: What is the block bored to? Standard 4.040" bore size or some oversize? 360 pistons are what come to mind but they are not available in infinitely large oversizes to fit into the larger bore 340 block.