You're either going to be a terrific junkyard digger or have deep pockets.. Either way, one of the prerequisites is a knowledge and skill with electrical things.
If you have a Clevor, you'll need a set of adapter plates to make the 351 EFI intake manifold fit. Or see
http://www.summitracing.com/parts/tfs-51611116 $777
The harness kits you buy new are probably easier to install, less cut and connect. They are also somewhat expensive -$525-$750.
A junkyard harness is going to be cheaper, but will involve a lot more labor. My experience with removing an EFI harness from a 5.0 Mustang was that it was a little like wrestling with a python. Once you get it out, you will probably need to cut or enlarge a hole in the passenger side firewall to feed the harness through into the engine compartment.
Computer:
A9L (5 Speed) computer from junkyard $100-$150
A9P (Auto or in a pinch, it will work in a 5 Speed car) computer from junkyard $100-$150
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Here's what the harness looks like outside the car:
Diagram courtesy of Tmoss & Stang&2birds
Main Computer harness this is for 1989 model 5.0 Mustang, and is the best choice for simplicity's sake.:
Engine mounted EFI harness for fuel injectors and engine mounted sensors:
O2 sensor harness:
Beware that the O2 sensor harnesses are specific to the type of transmission in the car. Use an automatic trans O2 harness with an automatic trans and A9P computer. Use a manual transmission O2 sensor harness with a manual transmission and an A9L computer. In a pinch and A9p can be substituted.
36 LB injectors, Calibrated matching MAF or custom tune, 190 LPH fuel pump.
I can post more information that you will need if you are really interested. Fuel pump & injector sizing, EFI harnesses from junkyard cars, fuel tank & fuel gauge information.