All sound advise you have received from fellow members but I might add a few points of interest.
There is fake R-12 out there so be careful if you buy some online.
There is nothing wrong with using R134 if you know what you are doing.
Condenser fins might look clean but they are not clean unless you use sodium hydroxide to get them shiny but again, you must have experience with this stuff because it will eat metal alive if used improperly. A condenser that looks clean but has grey oxidation on the fins loses about 30% capacity which can cause liquid slugging. Liquid slugging is the kiss of death to a compressor. The inside coil must be clean as well. In a properly maintained AC system, half the refrigerant should be in the condenser and half in the inside coil. If a coil is dirty then the system is thrown out of balance. Since the returning refrigerant cools the compressor it's important that everything is checked especially the inside blower and the expansion valve or fixed orifice. Vent temperature by itself means nothing if the blower is on low or does not move enough air because the squirrel cage fan is dirty.
If you know the temperatures of the pipes you know the pressure since R-12 like all/most refrigerants are predictable. Using a pressure temperature chart will tell you if the system has R-12 in it or something else. I would need to consult a factory service manual insofar as the oil level and everything else. Some oil mist circulates to lubricate the compressor moving parts but I'm not sure.
I know that a fan shroud and a thermostatic fan would be a good idea with AC on a early mustang.
