There are many threads about how to do it, who makes the kits and what works best.
Basically, if you just want add on a/c with hot air still coming in through the floor heater vent and defrost vents, go with the NAP kit. I have it in one of my M1009's and it works great. 38°-42° air at the vent on the hottest summer day. Duct tape helps keep the heat from coming in from the floor vents. Nothing short of full interior insulation and especially, insulating the fiberglass top will ever make the inside of the truck comfortable in the full Texas summer though. The NAP kit makes it so your front isn't sweating.
However, on a day like tomorrow is supposed to be with a tropical storm coming right at us. High humidity rain in the summer means you have the a/c on but also have to have the heat system on the windshield to keep it defrosted. You are still sweating in the truck with the a/c on.
The Vintage air kit replaces the stock heater box, removes all hvac stuff from the engine side of the firewall and allows you to have any combination of heat/ac at the floor, vent or defrost. It also cost more than the Nostalgia Air Parts kit. The Vintage air kit doesn't have the compressor mount just for the CUCV that the NAP kit has either. You can get it from NAP or from their supplier in FTW for a few hundred dollars though.
I meant to just write a few sentences and tell you to search. Oh well. I am going on my 4th summer of a/c in my M1009. It is better than nothing, but without insulation and probably tinted windows, not anywhere near what most people think of when they think of an air conditioned vehicle. Nothing can keep up with the green green house effect.