Well, you seemed to be determined to do this, so, good luck....
I forgot to mention, you will also need new lo and hi service valves, for the 134a......it's imperative that all the old R-12(which should be gone), along with the old oil(mineral oil), be removed......it will be replaced with 134a refrigerant and a ester/Pag oil.......this oil does not mix with mineral oil, thus a flush should be performed....I worry about 22 year old evaporators and condensers......cross your fingers....