were do i get valves for gt40 p heads

what kind of gains have you all seen out of the P heads? My brother bought a crate motor that has the P heads on it, unfortunately no one makes headers to swap a P head 302 into a mustang II, so he had to pull the heads off and is going with some edelbrocks instead.
I've never seen any hard numbers of how much better the P's are than regular GT-40 stuff though.