too many people get caught up looking for mustang parts as if Ford made parts mustang specific, It's Ford guys - you dont have an Italian guy named Guido using an English wheel to make the firewall. An explorer engine will most likely cost you less and is basically filled w/ the same parts
NO not all explorers have pi heads. it depends on year, if you go off that assumtion you might end up with 3v heads or pushrod heads.. If it is 2v sohc then it's PI.