I did this on my '89 a few years back.
I had the compressor rebuilt with specific instructions to make it ready for r-134a. Stock o-rings are black and are for R-12, and so you have to replace all the o-rings everywhere for r-134a (they are either blue or teal in color). On my mod I bought a kit with several sized r-134a o-rings, and matched them up to the r-12 ones. Next you have to clear the system of the old oil. I used mineral spirits and forced air through every hose, especially the evaporator and the condenser. I have heard getting a larger tri-flow condenser works better for a retrofit. I then refilled it with PAG oil (around 6 ounces) which is supposed to help condition the system for r-134a.
Now the last part is the most important. Replace the dryer with one meant for r-134a, and with it make sure to get a matching A/C cycle switch. Using an r-12 switch is asking for trouble, as R-12 cycles at a lower psi.
Then replace the Schraeder valves with r-134a retrofit valves, draw a vacuum on the system, and then fill the system with r-134a. I stopped when the psi was between 35 and 40. Some other people say that 45 psi is nominal on a retro system. This worked for me. YMMV.
1989 Mustang GT (Daily driven, 310,000 miles, Bone stock)