I am just finishing up the efi conversion. I just came in from wrapping up the harness, and putting the plastic sleeves over it. I used a harness from an 89 5.0. I would say that it is doable if your are comfortable with a digital multimeter, and have a good knowledge of electronics. The biggest problem I found is that the color code shown in the wiring diagrams did not match my car.
That said, it can be done. I fired my engine up for the first time last week. It starts easy, and runs okay for an engine that has been sitting for 4 years.
If you have any specific questions, I would be glad to help you out.
I just finished up installing an 89 AOD into a 67 fastback. I would be glad to awnser any questions you have, I have all the Ford EFI shop manuals and I am Ford Cert.I can email wiring diag etc if need, this is a great conversion