Don't bother swapping to the 04 GTO PCM, it will just add more headache to what you're trying to do. Use the 02 OS 12212156 for a Silverado and start with that as your base file. If this is a swap, you can use a Camaro file too, just note the pinouts change. I *believe* the 00 and 02 Silverados have the same PCM pinout, I'd recommend double checking. Look at the DropBox for a base file, there are a few to choose from. Simply Full Flash this file into your 00 PCM and then begin the COS tutorial (replace .tun w/.ctz as Joe said).

Follow the COS upgrade tutorial from there, this OS gives you COS5 which is the best COS EFILive has for the 512kb LS1B PCMs.