I just started another EFI project for a friend. We picked up a running 96' Explorer 4x4 for $700. This is the most cost effective way to go to a newer 5.0/EFI, if you're able to do this type of work yourself. Just about everything you need is there. I send the ECU to EFIguy along with the info that he needs for a re flash. The harness I configure myself, but EFIguy provides that service if needed. The rest of the things needed for completion are the same weather you're doing OBDII(Explorer) or OBDI(Mustang). With the Explorer, it uses the 4R70w trans which is controlled by the ECU that EFIguy sets up. Coupling the 4R70w to the D20, is the same as going with the AOD.
For info about ECU flashing, PM EFI Guy on this board.
Mark