You could use a junk yard system but it would need to be a late model (R134) system. A 134 compressor would likely need to be retro-fitted to use the 70's v-belt drive and mounts.
A older R12 rear air system could be flushed for R134 use but you will need to convert it from a TXV controlled evap coil to a orifice system for proper oil return. And you'll need to use a suction accumulator.
Either way you need new hoses as they are specially lined to prevent leakage of R134. R134 molecules are tiny compared to R12 and can seep thru unlined hoses.
Hod Rodders have been using systems like these for years. Might search Vintage Air and others for their "hidden" systems. Not cheap but it still beats cutting a vintage dash up.
Just dig up one of those mayonnaise jars full of money you got buried in the back yard.