Retro fitting an AC setup.

This is certainly not a cheap alternative but it is a reasonable solution:

Complete Mopar A-Body Air Conditioning Systems | A-Body AC Systems
There are aftermarket hotrod type heat and a/c units out there too. The factory unit for pre 73 had a push button controller, a overly complex water control valve, and other components that did not withstand the test of time. Factor in its R-12, compressor, etc.., and a all new modern unit could cost less in the long run.