?'s about getting A/C Working on 75 Duster

Your car would originally have had a V2 compressor (9.45 cubic inch displacement), not an RV2 (10.5 cubic inch).

Those compressors are not "known to leak". Some other compressor designs such as the GM R4 are known to leak, but not the V2/RV2. If you have the system open and empty, it's an opportune time to install a new shaft seal as a sensible precaution against leaks, but other than that, if it's in sound shape (and they usually are; they're very durable), go ahead and run with it. It's a common misconception that axial compressors (e.g. the Sanden, Seltec, or Nippondenso) are "more efficient" than the reciprocating V2. They're lighter weight, for sure, but internal friction in the axial compressors is much higher than in the V2 due to the greater piston/cylinder total surface area (5 or 6 or 7 pistons instead of 2) and the losses due to the swash plate. The V2 requires lower overall torque for a given head pressure, though its torque profile is very "peaky" (torque input required spikes sharply as each piston hits its compression stroke), so the mount brackets have to be set up correctly and belt setup and tension are more crucial than with an axial compressor. I like to use the clutch assemblies with an inertia ring (flywheel) built in; they smooth out the torque peaks.

Do make sure the EPR valve is working correctly. It's located in the rear (low side) port of the compressor. Once the system is charged up, you can follow service manual procedures to put an eye on it and see how it's doing. Or you can replace it on speculation if you like. They haven't given me much problem over the years.

R12 or R134a are the only refrigerants you should use. Do not use the hydrocarbon-based "replacement" or "substitute" refrigerants (OZ-12, HC-12, RedTek, etc.). They work, but they're dangerous and illegal. Do not use the non-hydrocarbon "replacement" or "drop-in" refrigerants, either (Freeze-12, etc.). They work poorly and cause problems. R12 really isn't expensive; there's not much demand for it any more and your system really doesn't need that much of it. Go look on eBay and see the steady parade of NOS R12 cans!

The best upgrade you can make is a parallel-flow condenser. This will make an enormous improvement in the efficiency and performance of your system regardless of which compressor and what refrigerant you use. Lower head pressure (and therefore lower torque load on the engine from the compressor) due to lower flow restriction.

Reference: parallel-flow condensers
and comparative refrigerant tests under standardized conditions -- look at the difference in performance between a serpentine condenser like you have now, and a parallel condenser...with ANY refrigerant.

The other big upgrade has nothing to do with the workings of the A/C system itself: insulate the car! Find and seal all the holes in the firewall and floor pan, take down the headliner and put insulation up there before reinstalling the headliner, insulate the door panels, and put some quality tint (Llumar, for example, not the cheesy dark purple junk) on all the glass. This will dramatically reduce the "solar oven" effect that defeats even the best-working A/C system.