134A was only mandatory starting in the '94 model year. Some vehicles had it a year or two before that, some waited until the end. Ford waited. People piss and moan about 134A not working as well as R12, but that comes from putting it in worn systems not designed for it. I converted an '89 GT and it would freeze you out of the car in 90+ weather.
I converted my '91. Still has the original R12 compressor though. I just flushed the system, added the right compressor oil, vacuumed it down. I checked it for leaks at that point and then charged the system.
It'll freeze you out of the car and it took me less than a day to do it.