I have a 91 v6 manual and I am planning on swapping a 5.3 lm7 with a t56. If I am keeping the original harness and computer from the lm7 do I have to repin the body harness to fit the different ecu and what will need to be added for the t56. I know it's difficult to do the wiring yourself, but if I knew what needs to be done I'm confident with my experience that I can do most if not all of it myself.

 

In a general way, the ECM and body are 2 different electrical systems. There is minimal interconnection between them.

You'll need constant batt power and ign power to the ECM. Its harness contains wiring to all the sensors and whatnot that it needs; cam pos, crank pos, etc. It's easiest to find the wires in the harness at the fuse box plug, and make the connections to the car at that spot.

Other things you'll need to manage include the gauges and alternator. For the gauges you'll probably want to retain the car's original wiring, maybe extend a wire here and there to reach some transducer that's not in the same place. The ECM controls the LM7 alt but since you probably won't be able to use it as-is anyway (won't fit under the hood) its wiring will depend on how you handle that.

You can use your existing AC compressor wiring, again by extending it to wherever the compressor ends up. Again, the LM7 one won't fit your car without modifying the frame, which is not a trivial matter. No doubt, whatever FEAD setup you choose, will determine both the compressor and alternator arrangements.

For the T-56 you'll need a pulse converter box such as Dakota Digital to get from 17 pulses per rev to however many your speedo needs. Backup lights just plug right in. Best to remove the reverse lockout solenoid altogether and plug the hole.

See the sticky about LS engine swaps. What you're doing is extremely common and ordinary. You can find LOTS of info on the details and various ways of dealing with the obstacles. Read read read read read read read read and read some more before posting any more questions.