If you find the somewhat-rare (not all 91's came with them) 1991 needle cluster, you only need to move 3 wires around in the dash harness connector and the 1991 cluster will plug right in. Being sure you get one from a truck with same # of cylinders would be best so the tach reads correctly. You'll still need to calibrate the speedometer unless somehow magically you got one that came from a truck with the same size rear tires and same rear axle ratio...
You can put a 1992-1994 cluster in there but it's a lot more work. Full write-up is over on Fullsizechevy forum, I think that's where it was originally posted. More than a few people have done it but it's not a slam dunk simple job. Much easier to find that cluster though, like night and day, and calibration of speedometer is done by modifying the DRAC module instead of the cluster itself.
Either way modifying the 1991 cluster or the 1992-1994 DRAC to be configurable is about the same amount of work, but it's WAY easier to try different settings on a modified DRAC and compare to GPS for accuracy, since you can leave it hanging loose in the glovebox to change switches until you're done, but on the 1991 you'll have to pull the cluster apart a little bit each time to get to the switches.
Richard