I converted my '90 to R134a. As I remember I did the following:

-replaced all of the o-rings (at least as I exposed them) with 134a compatible ones
-cleaned up the evaporator and compressor and drained as much of the oil as I could
-replaced the receiver/dryer
-put PAG oil in for the 134a. I don't remember how much, but I wrote it down somewhere. I calculated/guessed based upon what the specs were for R12 morphed through whatever conversion factors I could find
-vacuumed it down and let sit for several hours to check for leaks
-filled it with 134a, once again I calculated the amount by seeing what was recommended when converting

That was a couple of years ago. I've had to top it off with refrigerant once. It works pretty well. On the highway it's fine or when the temperature isn't crazy. On those 105 degree Texas afternoons when you're sitting at a stoplight, well it leaves something to be desired. But to be honest, I don't know how great it was when the car was new compared to how accustomed I've grown to the AC in my more modern cars.

You can still buy R12 on places like eBay, but it's too expensive for my blood.

steve