@MCBMW Will your air compressor supply 4.2 CFM @ 90 PSI? That’s a whole lot of air. That’s what Harbor Freight says you’ll need. Unless you have a heck of a good compressor, I doubt it. Those Harbor Freight venturi things also shriek like a small jet plane.
I also doubt you can buy genuine R-12 near St Louis legally for $20 a can. If you can, sign me up. Freeze 12? No problem to buy, but there are others. One problem with freeze 12 is that anyone who might work on that car’s A/C system after you are done with it will suck contaminated refrigerant into their service equipment. Who knows what Freeze 12 + R12 + r134a add up to.
As for your compressor working with modern legal R-134a, never fear, it will compress it just fine. It doen’t care what the gas is. You will find that R-134a will eventually leak through the original R-12 hoses. R-134a hose has an internal barrier that is different from R-12. The molecular structure of the R-134a will allow it to eventually seep thru the old specification hose. You will probably have to recharge it every season or so even if there are no other leaks. If you do get a new hose made up using your old fittings, be sure they use the proper hose. If it were me, I’d sure take this opportunity to retrofit the system for the new refrigerant.