Hi there,
A/C were available on older Volvos as a dealer option. Mine is a 74 and I installed it at the dealer 1 year after I got the car. Well, mine came with the switches and vents as well as the blower, so all that was missing it the compressor and the plumbing. You could find a 164 in the salvage yard with an underdash blower or there were many aftermarket a/c with the underdash blower available around at that time. My 78 Mazda had an aftermarket a/c with the underdash blower.
Might I suggest something here. After 30 years, my York compressor quit, as it has been in continuous use (its summer all year round in Malaysia!). I had it replaced with a Sanden rotary unit....to tell you the truth, it was much lighter compressor than the York. It works well, and all it needed was a slight modification to the mounting bracket. A good welding shop should be able to to this at a relatively low cost, at least here in Malaysia. I have also installed an auxilliary push fan on the a/c condensor in front of the car and this helps the a/c to maintain its cooling ability even in bad traffic jams.
Good luck with your installation. I hope that you will be driving "cool" soon.

Dave