372or 416 and why

If you are a racer, than either one is fine. If you are a street driver, adn race intermittantly, the longe rthe stroke, the more street friendly that power level will be. In terms of 500hp with X/J/587/etc... All you need to do is throw money in them and they'll work. They can be made to go 280ish at tops, that's enough for 560hp with properly matched parts. The caveat is you still have a 50yr old chamber, and the associated weight and design of that family of head. For less money than it takes most people to get iron heads to perform the same, aluminums can be bought and you get the double bonus of weight and design, plus repairability. I've ported iron heads that were replacements for cracked ones after being maxxed out port work wise... So it becomes a what do YOU feel like deal. 450 is easy on a 4" engine with iron heads that move 240cfm. Thoase heads are stable, and within relative cheap cost (stage two porting with flow test) that will about equal RPMs out of the box. Assuming you are starting with core heads, having neverything they need done by a competent shop, and using comparable performance parts on them anyway. IMO, I'd go with a 4" crank, 450hp expectation and run the iron heads you want. If your goal is 500hp, then go 4" crank, dished Diamond pistons, and RPMs with the standard work. With the right cam on a modern lower end design, they can make 500hp.