The GT40P heads (mid '97-'01 Explorers) flow better than the GT40s. The sparkplugs in the GT40P heads are at a different angle than previous 302 heads. If you're using Explorer exhaust manifolds this is not a problem. If you're trying to use Mustang exhaust manifolds or headers the sparkplug wire boots will lay against the header tubes and burn. Headers for the GT40P heads are expensive if they still make them.
The GT40 heads ('93-'95 Cobra, '96-mid '97 Explorer) are the best factory heads without having to use special manifolds or headers. The E7TE heads ('87-'95 Mustang, '87-up trucks) aren't too bad either. Stay away from the E6SE heads, they suck.