I would recommend searching the numerous A/C threads here, because there have many different approaches and varying degrees (pun intended) of success. In general, some choose to do aftermarket systems, but blower in the cab = loud and they are recirc only. That can be a problem for Defrost, depending on the climate.
Alternate compressors do not have the displacement of the R-4, but the R-4 is also known to develop leaks and the quality seems to vary. If you do the modifications to run a Sanden (/Sanden style), be particular to get the biggest displacement you can fit. It will cost more, but why do you want A/C anyway? Likewise, parallel flow style condensers are theoretically better than the stock style, but many of them sold to "fit" a square body are smaller than the stock one, so the overall performance is similar. Check the dimensions, as sometimes there are ones available that fill the whole space in front of the radiator like the original did.
Also pay attention to the refrigerant, as R-134 does not work as well as the R-12 the old systems were designed for. I personally avoid it, but it you see it as the only "legal" option, try to make the hardware better than original to get similar performance. This pushes some people to electric fans, since the A/C still has good cooling at stop lights and low speed. A heavy duty fan clutch can also help. Research which size orifice tube (or V.O.V. if you're not risk adverse) should work with the combination of parts and gas you're choosing.
