Same here. When R12 hit platinum status, I read so many reports warning NOT to retrofit or mass destruction would occur!!! I figured that my system was dispensible at the time. If it breaks, I take it out and save weight, so I did the conversion as simply and basically as I could. The conversion kit box was about $39 at K-Mart at the time. Lo and behold! It works. No magic, no ghosts, no dragons, no wizards. It just plain has been working for eight years now. I think the R134a molecule is larger than the R12 which makes it tend to leak less. At least that's what I "heard". I had to file and drill the "conversion" fitting for the low pressure charge port but other than that, the boxed conversion kit fit the bill. I'ts not as cold as R12 but it's cold enough that the system cycles on and off properly to maintain cool cabin temps.
MOD:
I wired a manual switch in the HVAC circuit to turn on the compressor in VENT, DEF, HEAT, BI-LEVEL modes. This allows me to defog the windshield and get FRESH air with A/C if I want to. Make sure you keep the thermo switch in series with the manual compressor switch so the A/C can still cycle. I also wired the e-fan to come on whenever the A/C is on.
zcarnut probably describes the right way to do it but, I did it the backyard way and it still works ok. I think the Z compressors are pretty tough. They are definitely stout and heavy compared to modern units.