The 86 5.0 has or should have the crappy E6SE heads. Pistons will not have valve reliefs, so swapping heads and cams later can cause problems. The E6SE chambers look smaller but they're also deeper, moving the valves farther away from the deck, this is the reason the pistons didn't have reliefs. The valve seat is about 1/8" deeper in the head. These heads generate 25 less hp than the later E7TE head.