Welcome to the forums. Good luck.

 

You need the whole harness and ECU. You'll also need the V6 dash cluster swapped. Also, remember to swap over the ignition so your key matches for the ECU. You'll also want the exhaust stuff off the V6 and the front suspension. V8 springs are rated for the added weight, and the front of the car is going to sit about 1" higher with the GT springs under it.

The motor swap (V6 to V8 or V8 to V6) is more involved than most think.