greyfox69 wants to convert his 88 5.0 to carb. PM him and tell him what you know about your carb experience. Maybe you can trade carb parts for EFI parts...
The 4 cylinder wiring harness won't work. It is very different form the 8 cylinder wiring harness.
Mass Air EFI systems are more adaptable to various engine modes without argument from the computer. For that reason, I would recommend a Mass Air conversion.
Mass Air computer will be an A9L or A3M (5 speed).
An A9P (auto trans) will work if you can't find anything else.
You have 5 wiring harness options:
89 Mustang 5.0 harness for a Mass air setup.
Ford Racing Mass Air conversion harness M-12071-A50* $575 at Summit
Ford Racing M-12071-A50 - Ford Racing Multiport EFI Wiring Harness Systems - Overview - SummitRacing.com
Ford Fuel Injection Mass Air conversion harness See
Ford Fuel Injection » Wiring Harnesses and
RJM Injection Tech — Welcome harness is $550
Lots of good information on the two sites above. They have a page that tells you all the parts you will need besides the wiring harness.
Junkyard 86-88 5.0 Mustang Speed Density - can be converted to Mass Air with an additional adapter wiring harness. Or you can use is as is for Speed Density.
You will need a different distributor, fuel pump, fuel pressure regulator and fuel lines. You may benefit by buying a wrecked 5.0 EFI Mustang and stripping it for parts. Check with the insurance companies: they sell the wrecked cars for a very cheap price. Some will go for $250-$400. Sell the shell for scrap once you have stripped and sold all the usable parts.