[Coolbreeze]

You don't need to swap the harness, To make an non-HO to an H.O you need:

1)Swap the harness if you want mass air
2)Swap the EEC
a) If you don't swap the engine harness for mass air get an speed density EEC
b) If you swap to mass air get a mass air EEC .
3) Swap the cam
4) Change to 351 firing order
5) Swap injectors to 19's or bigger with match maf(if mass air)
6) Swap heads to E7's
7) Swap intakes, you want one from an 87-93

I did this swap on my brothers 88 T-bird with a 302 with all my stock 88GT parts.