Headliner isn't too bad. I did it while waiting for my son's Football practice one evening. They don't actually fit. They are cut too small. the 2 piece cardboard ones from Jim Carter et al. The one piece fiberglas are supposed to fit better and be fairly easy to install.
The GMC guages have a higher oil pressure to them due to the higher pressure in the GMC motors. Scott's speedometer service could set you up with a cluster that's ready to go. I bought an NOS one some years ago for short money.
Check out hisa phot gallery. There are 4 Deco GMC speedo's on the shelf shots waiting for an owner.
http://www.scottspeedometer.com/photogallery/
I know he has complete clusters as well. Good people. Knows his stuff, been doing it for years. His sons are in the biz too.