I first tried to resurrect the missing A/C in my '75 with a rebuilt factory A6 compressor, but found out later that the A6 frequently suffers from "black death" and complete failure (which happened to the rebuilt unit). However, they now sell the Pro6Ten, which has the same form factor as the A6 (uses same brackets and hoses), but is really a Sanden in disguise. However, you may not want to track down all those A6 brackets if they are missing.
I'm currently using a Sanden compressor and brackets from Vintage Air, and had custom hoses made to connect it to the stock evaporator and aftermarket condenser. The rest of the system is still the stock GM parts. Its kind of a PIA to keep it all working right, since the nest of vacuum hoses and vacuum actuators under the dash sometimes go haywire, but the system does work well to heat and cool. I used R134a and PAG 100, new receiver/drier, green O-rings and a Ford Blue orifice, and charged it to about 75-80% of the R12 recommendation. YMMV.
I've currently got many of the underdash parts out because an $8 "diverter link" on one of the doors inside the plenum behind the dash is broken. Its the link to the vacuum actuator that moves the door between defrost and heat, so I'd like it to work properly. However, there were quite a few pieces to get out of the way before I could take out the one screw that holds the door link in place, and I'm waiting on lmctruck.com to mail out the new link. I hope it matches the old one.
You can pull the glove box door and the 3 screws that hold the plenum cover just behind it, and then you can see into the heater box. You may find a mouse nest and/or all the pens, match books, bolts and clips that have fallen into the defroster vents. At least one of those bolts is probably laying down and oriented so it keeps the heat/cold blend door from moving fully in both directions.
Bruce