I'm doing the same swap a d I know this is old but I found a plug that looks like this at the engine side of the fire wall right where the main pcm harness goes thru the firewall.
Did you get it working? I'm in the process of LS swap on a 98 k1500 with push button 4x4. I'm trying to figure that out before I have the wiring all done so it's easier to trace what wires I need.