Explorers between 1996-2001 came equipped with some sort of GT-40-ized 5.0L engine, but with cast iron heads. Read this thread for the most complete info.
Explorer engines had cast iron GT40P heads (better than GT40 heads on the Cobra and much better than E7TE heads on the GT) and an intake manifold almost identical to that of the Cobra. Unfortunately they had a much milder cam than the GT/Cobra Stangs and that, together with a more restrictive exhaust, limited HP/TQ outputs from the Explorer 5.0.
1996-mid '97 Exploders/Mountaineers came equipped with GT40 iron heads and cast GT40 intake with fox-style internal EGR provisions
Mid-'97-01's came equipped with GT40P heads and no provisions for internal EGR (SN95 style)
P heads will require either FRPP or MAC P specific shorty's or BBK or MAC LT's can be used with 90* plug boots.
GT40's are identified by 3 vertical bars cast into the ends of the heads, and "GT" cast beside the lower rear head bolt hole. P heads have 4 bars and "GTP" cast in the same locations.
Neither have smog pump provisions, and both will require a valve spring upgrade, even with the stock HO cam.