It really depends on the combination, but for 550-600hp you certainly don't need P51s, in fact if you know these engines well enough you can do that power level on pump gas with factory iron heads.
If you simply must step up to aluminum, then at this point I'd recommend the Ford Motorsport SCJ heads. They are essentially what the P51s are derived from but cost a lot less, are plentiful, and are always readily available should you need a replacement for some reason. Keep in mid that with either the P51 or SCJ heads you will need pistons with a different valve layout for your setup so a bottom end rebuild is part of the equation.
This head recommendation might change depending on further details about the build.