Doing this on a 1988-1994 WAY easier than 1995-up, simply due to the difference in complexity of wiring harnesses (although 1995 is fairly simple engine harness, dash harness starts getting stupid plus both are one-year-only) and that dropping the dash on the earlier interior is easier.
You'll need a donor truck from a very similar year/engine, for all the parts plus wiring harness. Haven't looked at the dash harness on the older trucks when non-A/C (have never seen a single one of these in person) to know whether the wiring for the controls and actuators will be in place.
You may need to use a hole saw to cut the firewall holes for the evaporator core fittings. You'll need the HVAC box for behind the dash. Engine harness (and as mentioned maybe the dash harness), controls, A/C compressor, hoses, condenser, accumulator, and *maybe* the accessory drive brackets which you would do well to get from a 1990-1995 so you get the better 2-piece aluminum setup vs. the crack-prone 1988-1989 steel bracket. Don't overlook all the support brackets that keep the thing stable. Take pics before removing.
Oh, that's about it.
I haven't actually done one of these, but I did look extensively at doing one for a customer's later model fleet truck. Hard to imagine AT&T sent their field reps out in TX heat in non-A/C trucks; almost have to think it came from an auction somewhere else instead of local. I came up with a parts list and a price that I thought was pretty fair but in the end he bailed on the idea. It's not a small job.
Richard