If i were you, i;d switch to the new 134 freon, but in order for that to work properly, you will need the new condensor for that specific freon. I have been fighting my ac system for almost 3 years now, doing whatever i can to get it as cold as my other truck which came from the factory with it.
I asked a few ac shops around town here and thats what they were telling me too, the older r-12 condensers don't have the cooling capacity that the newer ones do. A new condenser that is made for the new freon was nearly the same price as a stock one anyway.
When i did the ac in my old Monte Carlo several years back, i had to buy a new compressor,lines, the system needed flushed and then charged. I had a ac service place do the flush and freon install with r-12 at the time. After everything was bought and installed i was runnin aroung $500.