The trannys are different. The GTP comes with a HD or heavy duty tranny. The blocks are the same but the heads and intake are different.

you will needs a wire harness and a gtp computer.

The GTP also comes with a bigger radator.

Now your stock tranny (assuming you have the 3.8L not the 3.1)will bolt up but it will NOT last. Even the GTP tranies dont last long if you go over 300 HP.

For some rason the people who put turbos or Cintrifical (SP?) SuperChargers on a l36 motor have better luck with the trannys than those who do the swap. Plus the l36 trans has a higher gear ratio which is better for the track.

Another note: The M90 isn't effeciaent at all. check out zzperformance.com or intense-racing.com for a Turbo kit or CSC kit. also might want to look at L67swap.com

If i was going to start over, id toss the M90! hope this helps.

Good luck!