You will need to pick up a gauge to measure how much R134 you put into the system. R134 runs at a higher pressure than R12, so you use about 20% less of the R134.

From what I understand you will get better cooling when using an R134 condenser and not the R12. I changed my '90 over to R134 in '98, and ran it for about 7yrs. At the time I made the switch R134 condensers weren't that easy to come buy, now 11yrs later you should be able to get one relatively cheap from a wrecking yard. IME w/R134 in an R12 system wasn't a good one. On Days over 95* it was useless, and at most ot would only cool the cab off to the mid '80's. In '05 I switched over to Maxi-Frig vs R12 and haven't had any complaints since. At the time I made the switch, my R12 comp was making a pretty bad noise on start-up, but has worked fine(no noise w/the Maxifrig). The one downside of Maxifrig propane based refrigerant, which tends to scare people away from using it.