As Ron advised, pull the entire engine bay & engine control module wiring harnesses from the Mustang, along with all the sensors.
You'll have to construct a high pressure fuel system & fuel return system for your car. The fuel pump MUST be fed by gravity, or be located inside the fuel tank.
Luckily, the engine management system in that '89 Mustang is Ford's EEC IV, the most well documented EFI system for retrofitting there is. There are several companies located in the USA that specialize in this, such as Quality Roadsters. Windsor Fox Performance Engineering is another such company. Ford's performance parts division (i.e. Ford Motorsports)used to sell a wiring harness, the sensors & a booklet themselves, but I no longer see those items in their catalog. You might find them on e-bay, or on a vendor's shelf. There are several books available with the info you need, such as "Ford Windsor Small Block Performance" by Isaac Martin, HP Books #1323. Chapter 7 of that book is dedicated to making the EFI swap into a non EFI car.
One gotcha to be aware of, the engine control modules for cars equipped with automatic transmissions and manual transmissions are different, so if the donor Mustang is equipped with an automatic transmission, you'll need a different module.
I assisted a friend in swapping an EFI system into a classic Mustang more than 10 years ago, there were no kits and little info available at the time. We were still able to muddle through the conversion, and afterwards, the car fired up on the first try and ran like a top. That is the beauty of the Ford EEC IV EFI system.
Thomas Tornblom's web site may also provide insight, he retrofitted EFI into his 1984 GT5 Pantera:
http://www.detomaso.nu/~thomast/efi/Of course, those of us here on the DTBB will help any way we can.
Your friend on the DTBB, George
