I am currently doing an EFI swap in my 1968 mustang Fastback. Have an entire 1991 mustang gt 5.0. Is there any company, in which you can send them the 1991 mustang harness and can modify to fit on the 68 mustang.
i had a diagram for 86 cougar /mustang factory efi harness but i am not sure if the wiring would be the same for 91. it uses the engine harness only and there is only 4 wires plus the fuel pump wires that run the whole system . i did a conversion back in 86 by tracing the wires and making a diagram. i put a
5.0 HO in my 78 ford truck . i will check and see if i still have the diagram.
Get the book: how to inderstand, service, and modify Ford fuel injection& electronic ignition controls. Its a little daunting at first glance, but once you study the book a little and with the help of the diagrams it'll ease out on the complexity. I took a brand new harness and dont see any point in sending it to someone to modify. Do it yourself so you can learn how it works and you'll find its not all that bad. going to have to upgrade your alternator and do an egr and canister purge elimator kit which you can find online or try ron morris performance....
I have looked every where and i can't find my diagram but i can tell you it isn't hard to use the original EFI harness if you can figure out the correct wires.
All of the diagrams you need are here: http://www.veryuseful.com/mustang/tech/engine/ I used that site to modify a 89 Grand Marquis EEC harness for MAF and incorporated it into my stock 66's harness which I rewired to upgrade the horn and headlight circuits. I ran one wire at a time and made the schematic as I went along.