I did it on my 71 with factory air. I replaced the hoses and receiver dryer, flushed the condensor, evaporator and hoses. I reset the POA valve to exactly 26 (see GREAT link below for the easy way to do this). I also drained my A6 compressor of all of its old mineral oil and flushed it several times with Ester oil then replaced it with 9oz. of Ester oil divided between the compressor and the dryer. I pulled 29" of vaccum for an hour, let it set all night and verified I had no leaks, vaccumed again for 20 minutes then charged system with 3 cans of R134A. I keep a little thermometer in one out let and get between 45 and 48 degree vent temps going down the road! I am happy with it! Check out ackits.com, they have a lot of great info that pertains to our trucks. I would be happy to answer any other questions you might have, PM me.
http://www.ackits.com/forum/messagev...&threadid=7567
Jeff