Is the engine stock or what mods have been done? The stock computer, in MAF configuration, (A9L for manual transmissions) can compensate for more modifications than newer computers. I’d search for the MAF conversion threads here.
Use an a9l or a9p and maf conversion harness. It is all you really need. I converted my 87 over to mass air using the maf harness. had to move a few pins on the main plug and wire in the mass air wires in, It wasnt hard and didnt take long.
So at this point I’ve gotten myself in a situation. I thought I would just need the ECU from an 87 5.0 since my car is an 87 4 cyl but I’m swapping to a 91 GT drivetrain. I bought the car from a guy who has done the entire swap but never finished the wiring. So I ordered a ECU for a 87 5.0 online for 130 dollars but the engine that’s been swapped in already has MAF. So now I’m wondering if I’m out the 130 dollars or if I could switch the MAF to speed density and what would be the easiest way to go.