Really no big mystery to the swap. The 2001-2002 3500HD's are simply GMT400's (GMT455's technically) with the 8.1. You would essentially swap everything to your project truck. Engine, PCM, harness, etc. All the little stuff like the TAC module (gas pedal, assuming these are DBW), air filter setup, those things. You'd need to source a different transmission as the 3500HD will have the extended tailshaft with drum parking brake on the back, IMO might be tricky to fit under a lighter truck. But, a year-compatible 4L80E from a 2500-3500 should be trivial to swap in place. You'll have exhaust work, driveshaft work, and whatever other wiring challenges you might face if you're trying to swap this whole works into an older (for example, OBD-1 truck.)
I'm surprised to see OP's comment "Even the parts department and owner are having issues finding information on these trucks" - just go online and find one for sale that lists the VIN and punch that into your parts lookup. Here's pics of two I dug up many years ago that the VIN is legible; a 2001 and 2002. Hope they help:
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I see OP needed a bit more info as initially he was planning to source the engine from a GMT800 so the accessory drive and supporting parts will be completely different. It probably goes without saying that the "easiest" route for this swap is to start with a 2001-2002 3500HD donor, instead of getting the engine from a GMT800.
Richard