There is nothing really wrong with the compressor except that is saps an inordinate amount of power from the engine and it is noisy. The York/Tecumseh compressors are prety stout, checp rebuilt and stone-simple.

The rest of the system is just not sized or designed for actual places that have summer.

Your best bet is to find some of the old threads on the various replacement parts (hoses, o-rings, schrader valves) that will be shot after years of non-use and age. Flush the condensor and evaporator and replace the compressor with a rebuilt one (because they are SOOO cheap). Of course an overnight vac of the newly assemble system is a must.

That is your baseline and you can decide which refridgerant to use from there. I will not go on my rant about hydrocarbon refrigerant (works great FWIW) but there are R12 replacements that are legal in the US, Recycled R12 is available legally and R134A is available.

My wife's 1981 245 with the same system as yours was recharge with Freeze12 when we just wanted to see if the AC system even worked...no flush, one schrader replaced...nothing else. It was blowing about 44 F at the vents idling on a 90 degree day. Not stellar 1970's GM AC performance but not bad. The car got taken out by the tree limb before we could test the long term effectiveness but with the steps listed above, likely you can acheive decent performance for under $1K.