Maybe look at replacing the o-rings in the A/C lines...
If you are going to convert to R-134 A, you need the green HNBR o-rings that are PAG oil compatible... They use PAG oil in the R-134 A systems to lubricate the compressor, so you have to make sure that all of your o-rings and foam seals are PAG oil resistant or they will disintegrate....
You should be able to use R-134 A in a R-12 system if you update the o-rings and suck it dry and use PAG oil for the compressor, the R-134 A will not run as cool as the R-12... Maybe update the receiver drier...