I just did the swap, it was very easy.
Great link for older harness:
Ford EFI Swap
Wiring:
http://www.veryuseful.com/mustang/tech/engine/images/88-91_5.0_EEC_Wiring_Diagram.gif
In 92 they switched to a slightly different setup.
Let me know if you need any assistance. The swap took me about 8hr to plan the wiring, do it, run the fuel system and put down the lower intake (I used stock engine and custom made chip for the firing order etc). Started on first crank, runs great around the neighborhood. I need to tuck some wires away then I'll drive it to the shop and weld in some O2 sensors and put it on the dyno to tune. Until then I am running it in open loop.
Wes