Here's what I would recommend:
1. Ditch the pushbutton 4x4 case for a lever case. This would be much cleaner, reliable and faster in your application. Keep with the GMT400 style lever case so you can use the speedo drive (rolls in to the next paragraph as to why). You need a couple things to run the pushbutton that you want to do away with. IE: Control module, front axle disconnect, etc. Without a front axle actuator, the transfer case shift module is just going to blink the button lights at you when you select it. When it doesn't see the proper signal from a front axle disconnect, it says "no way Jose" and goes full stop in the shifting process. You can get around it by leaving an actuator wired up in the system somewhere, but that is just going to look "hinky" on a show vehicle....
2. Gauges. Well, memory is a bit foggy, but I do believe the gauges are run by the PCM (for the most part) by the PCM. IE: fuel level, speedo, tachometer, etc. If you want to use a "stock" cluster, you're probably going to have to get creative and start swapping guts around. IE: for speedo, I'd get a diesel cluster since it doesn't use the PCM for signal but you will have to wire in a VSSB/DRAC moudule. The diesel cluster also drives things like the fuel gauge, temp gauge, etc from sensors and not through the PCM.
If I were in your shoes, I would probably grab a diesel cluster and pull the face off it. Then I would take a gasser cluster and stick the diesel gauge stepper motors on the gasser face (use the gasser tach), slap it all back together and then wire in the diesel senders. The only issue would be trying to drive the tach. Maybe use the diesel stepper motor on the gasser face as the sweep is the same with different values. The diesel tach stepper motor gets it's signal from the alternator windings, not the crank sensor. That would be an experiment you would have to just deal with though...
3. A/C. Should work. Not sure how the vortec truck run it though. The only thing I can see in questionable operation is the WOT cutout for the A/C. It's pretty much stand alone otherwise IIRC....
Now, here's what i would do with the truck you have: ditch the remaining gmt400 stuff all together.
Swap in a super reliable NP205, lots of aftermarket support. Build a custom gauge panel and go all Autometer or some other higher end choice. Just ditch stuff like AC,cruise, etc. it's useless on a show truck or mud runner anyways. That's just what I would do if I were building a show truck and not a driver....
You've got a bit of a Frankenstein there. No matter how you slice it, you're going to have to break a few eggs to finish your omelet.....
