Did a search on here but couldn't find what I was looking for. I have a 99 Trans-am that was originally a 6speed car that's been sitting for awhile waiting on a new motor and tranny. I found a good deal on a LS1 and 4l60 setup. I know I will need the PCM from the auto car but will I need the harness as well or will the M6 harness I already have be fine?

 

You simply need to do a complete reflash of your PCM, but you will need the harness from an auto car because the 4L60 has much more to control. The Pcms are the same and can be fully reflashed, but going to an auto it would be much easier to swap harnesses so you'd have the proper connections.

 

ND2 is right. PCM just needs to be reflashed. If you didnt want to replace the entire harness you could just run the wires for hte trans, but there are 13 wires. All go to the trans but 1 which goes to switched power. Probably easier to just swap harnesses. Also VSS connector is different.