By donor car I mean the car you will use for parts, strip all the parts you need from it and then take it to the scrap yard. I used a 1988 Crown Victoria Police Car. Many people use 89-93 Mustangs. The Mustangs have a Mass Air EFI system which is better suited for future engine modifications since the computer compensates for the changes. Crown Vics use a Speed Density EFI system which by its design cannot compensate nearly as well for changes such as a different Cam, heads, intake, etc.
I like the the crown vic setup because 1. they are easier than mustangs to find cheap, and 2. because the intake points to the drivers side making the throttle cable very easy to route and hook up. The mustang intake points to the passenger side but I haven't heard of anyone having problems with their mustang throttle cables, I just chose the crown vic because I knew where I could get one at the time. I converted the Crown Vic to Mass Air since I'll be installing a new cam, heads, intake, throttle body, injectors, and mass air meter soon.
The computer system Ford used in the 1980's and 1990's was called EEC-IV. I suggest you google for EEC-IV or EEC to find out more about the differences between Mass Air and Speend Density and also to find out which cars each system is found on. If you have not swapped an engine from one type of car to another, or two cars with very different wiring harnesses and different underhood layouts, I highly suggest getting a donor car. That way you can pull everything you need off and if you have room to store the donor car during the swap then you can always go back and look at it to see how things were hooked up before you stripped it.
Get wiring diagrams for both cars before you start. I spent most of my time during the swap working on the wiring harnesses and studying the diagrams. This can be the most intimidating part if you choose to do this yourself, but it's not too hard if you work though it slowly and methodically.
email me if you have further questions