Charlie is right. Properly sized components using 134 will cool as well as R12 (you wont be able to tell the difference).

Go to Vintage Air's site and there are some tips. Bigger condensers are the usual suggestion.

Although 134 is an interim coolant, the replacement is not yet a given. LIke Charlie says, 134 is cheap, available, and a known entity. Stay away from the Mexico stuff that has unknown gasses added....miracles may happen but they sure dont happen in thermodynamics.