Here's the info:
Any 97-07 engine and trans combo will work. It's slightly easier 98-02.
Here's what you need from the GTP donor car
Here is what you need from a 98-99 monte carlo Z34/Lumina LTZ as it originally came with a 3.8 and the wiring harness is very similar to the 90 GP.
Engine wiring harness (C100 at firewall will need slight re-wiring to work in the 90 GP and add the BBV wiring(I think))
tensioner, and heater hose fitters(they are at a right angle vs straight on the newer ones, but the newer ones can work).
power steering lines
AC components, everything from firewall forward
97 is odd because of the transmission and vacuum hose routing.
03 is odd because of the transmission wiring. If you use a 03 transmission, you MUST use a 03 PCM.
04-07 has a GEN V supercharger, you'll need a throttle body adapter to use either an L67 TB or Northstar TB and LQ4 MAF. Whatever TB you use, you'll need to match the TB Adapter. Additionally if you use a 04-07 transmission, you MUST use the 03 transmission.
Axles: If using monte carlo transmission, they should bolt in not issues. if using GTP transmission, take the inner tulips that plug into the transmission, and swap the rest of the 1st gen axle into those tulips. They will pop in with no issues, assuming the original 1st gen trans was a 4spd. If 3 spd, it won't work.
If I had to do this again, I would find the 98/99 monte carlo z34/lumina LTZ and steal all of the parts from. I would then top swap the 3.8 and run it in the GP.
Good luck with your swap!