maybe I can help
I have a wiring diagram of a 2000 Camaro and I suspect it is identical to the 2001. I'll be glad to send it to you. I also have a little experience with your headaches. I put the drive by wire 3.8 L w/ 4L60E from a 2000 Camaro in my 50 Chevy truck. There are many issues with trying to mix wiring harnesses. I would go completely after market or completely GM original and cut out the un-need stuff, but not mixed. I know Painless makes aharness to run the engine and transmission and it uses their own PCM. If you already have the Fast harness for the engine, you probably need to see if they also offer a harness to run the transmission. Did you Fast harness come with a PCM or does it use the OEM? A big problem with the factory harness is that many functions of the gagues, and interior electronics (door locks, hatch, seat belts and many other things) are run by a body controll module which has about a dozen wirtes that go to the factory PCM. To bypass the body module, will require a vehicle security bypass or a PCM reflash to delete the VATTS. As for the gagues, you can run any gagues you want, they just need the correct sensors, but the PCM will also need the factory sensors for many of the functions (water temp, knock, MASS and several others). The speedo can be run either electronic (new electronic speedo) or mechanical (adaptor for mechanical sender installed in the transmission). There are many issues here and the best ways I know to help, is talk about each as you wnat to address them. If I can help, please let em know. Keep trying, it only takes time!!