I've done this conversion on my '91. Bought a cleaning solution that's made for this conversion, bunch of 134a rated O rings, couple of fittings at Pep Boys. I had a leak in my a/c system so the sysetm was empty of R12. Old oil out of the compressor was emptied. Using compressed air (set to 15 psi), the lines and the condenser were flushed out with the cleaner. Took the leaky hose to an A/C shop and had another one made with new fittings for $30. With a new air drier, I went to the A/C shop to get recharged with R134a. The shop properly evacuated my system then recharged it with R134a and oil.
You can buy an air drier that comes with higher presseure switches (low and high) for the R134a from LR or other vendors. I chose simply to purchase a drier w/o any switches and transferred my old ones over.
The system works nice and the air is cold. There isn't any problem with the compressor cycling on/off and everything is fully operational. This is a pretty straight forward DIY project.
Ali