Note the 'bracket' some are speaking of can be either the huge steel plate bracket bolted to the front of the block & heads and mounts the alt and the AC compressor, OR a small u-shaped adapter-bracket that is used to run a Sankyo-Sanden AC compressor in a Pantera. The big bracket is Pantera-specific (two versions) while the small adapter-bracket is generic and was once included with Sankyo/Sanden compressors sourced from dealers. Making one from 1/4" plate is a 10-minute fabrication.
On your OEM York compressor, there are freon shut-off valves on top of the assembly. If someone has shut them off, you will have no A/C no matter what you do to the poor thing. The York is very heavy, shakes the car at idle and is an early '50s design, but they can be made to work.
As for swapping to r-134A freon, the OEM evaporator valve under the dash has a relief spring inside that controls the system pressure. The settings (or springs) are different for the two freons. Second, in the OEM setup, there is a fine conical screen in one leg of the same valve, designed to catch trash and ground-up dryer material from the can in the trunk. The tiny screen is usually plugged solid so no freon (either type) will flow. Hook it out and clean it, then return it and replace the drier. With r-134a, the screen is usually left out for more flow of the inherently poorer heat-transfer of r-134a freon.