I am currently in the middle of putting an L67 in my car. Soon I will be picking up a 4T65e-HD out of a 1998 GP GTP. I am installing the the powertrain into my car that came with a 4T60e. My question is do I need a new PCM, and are the wire harnesses the same between the two transmissions? Any help would be appreciated.

you are going to have alot of problems going to the 65, i believe that every trans is unique to the year, and trans are not compatable. It would be much easier to just go with a 60 HD

The wireing harness are defintely not the same.
Be careful about that grand prix 4t65-hd. I believe the transmission cases are body style specific? (either W-body or H-body like you) And I have no idea if the axles are the same length from body style to body style.
Your going to be better off using the computer and wiring harness from the 98 GTP. The 96 pcm isn't supported by major vendors because of the 4t60 tranny.

Both of them are right. Not only that, but the GP trans is a different final drive ratio. That'* even more PCM problems.

www.l67swap.com

www.3800pro.com

Between those two sites you should get most of your answers.

Getting a comprable PCM for that year GTP will be your best bet. I believe all Series II L67'* were 2.93 or 2.97 from the factory.

You will need the H body axles, that'* for sure.

Thanks for your help, after lots of research I have decided to go with a 4t65eHD out of a 97 Rivieria, because it bolts up, and the GTP trans does not. If anyone needs a low miles 4t65eHD out of a 98 GTP let me know, I know where one is for cheap.

Whoever told you that was smoking crack. The Riviera is also a W body, just like the GTP, and both those transmissions bolt up to any 3800.