Retro fitting an AC setup.

RedFish said
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.
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Not an AC expert, but shouldn't I be able to convert the R-12 to 134a. Since I have to make my own AC lines, I can control what fittings I use, VIA R-12 fitting on the end that connects to the unit in the dash and 134a fittings to connect to a new dryer/condensor and compressor. Or is this a huge no-no?