You can gain with aluminum heads, Arizona Throttle Body, and Cam Change. Then in needs to be tuned. Also Fuel, Injectors. Even with the stock 502 if you kept the bottom end, you can still make an additional 75-100 HP with these upgrades. I would say 575HP-600HP. To do it correctly and actually make the HP, certainly takes some coin. If you go 540 on the bottom end, and upgrade everything including cleaning up the intake and having them through honed and polished, matched to the heads and throttle body. 668HP is the best I have seen, that is fully dressed and dyno'd. at 5500. At 5501 rpm, it falls flat on its face and the Intake limits....not enough air. This is also running 92 non ethanol fuel

Id leave it alone, some after market heads have raised exhaust ports which can cause alignment issues with your pipes going through the transom.