I did it once about 18 years ago. It was pretty straight foreward, but I had a donor car with a complete system parked right next door, which made things easy. The worst part of haveing A/C in a pre 73 140 is that you lose the glovebox. The evaporator/blower unit bolts up in place of the glove box. There is a duct that runs behind the panel under the dash. The fuse box cover gets replaced with a panel with a vent. Another vent is cut in by the hood latch. If you can find a car with an intact system, it's easy. If you have to put it together from parts, it could get confuseing. I doubt that anyone carrys aftermarket kits anymore but who knows.