There are significant differences over the years in intake manifolds, valve covers, throttle bodies, EGR configuration and probably other things I don't know about or can think of at this time. The differences would have to be transferred from the original engine to the replacement engine. Its enough work to discourage many Explorer owners from purchasing a different year or make of engine. That makes less demand for SOHC V6 Mustang engines for Explorers which keeps the selling price lower.