There are a couple of options.

The aftermarket route listed above.

Stripping the factory air from a Chevelle/El Camino (not sure but I think 68-72 will work). You will need to use the factory air plastic dash housing (guage cluster). You also need the AC controlls for the dash. This is what I did with my 68 El Camino. It's a pretty fair amount of work and also includes dioing some firewall mods.

If I were to do it over the thing I would do is find a nice older under the dash unit in a wrecking yard (they usually don't go for that much) and and is probably the cheapest and easiest way to get AC in your truck.