The stock fan will double as a condenser fan, so you won't need that. If it runs too hot with ac, then you can put a pusher fan on it and wire it to the ac clutch circuit to come with the ac. You will need on the other hand an evaporator, the heater core like unit inside the truck. It installs between the blower box and the heater core/air distribution box. It usually requires a new evaporator box to fit in, the trucks without ac just had a box to take up the gap, and I don't think an evaporator can be added to it.
You'll want to get new o-rings for all the fittings, so while you are there get the r134a retrofit kit. That is unless the donor is already r134a. Last time I checked with parts, r134a was $1.00/ounce as opposed to r12 at $3.00/ounce. And it is more enviromentally friendly. . .
You will also need the a/c switch, associated wiring/relays. Also needed would be the a/c idle up valve to raise idle speed when the compressor clutch comes on. I see those on ebay all the time. Oh yeah, don't forget the compressor bracket.
Best bet is to find a donor truck and get all the parts at once. Try to stay away from systems that have been open for a while, dirt and moisture can accumulate in the system.