Some things to consider. A quality conversion to R134a can be accomplished but you need some R134a specific components. The condenser is different and the H valve or expansion valve is also different. The addition of an electric fan for the condenser also helps to keep the high side pressure down when sitting in traffic.

I did the conversion on my 92 D250. I used a R134a compatible condenser, a newer R134a H valve, a new compressor, evaporator and drier. Others maintain you need to change the hoses to the newer barrier style but I didn't do that as they are not available. Yeah I know I could have had them made but my hose ends are not in great shape and I didn't want to mess with them. So I flushed the hoses really well and used PAG oil because I work at a dealership and there are plenty of half used cans of the stuff sitting around. I also added two 8" electric pusher fans on the condenser.

Either way it works well, will cycle the compressor if it's not on high at around 38*. In the 5 years since I did the conversion I have not had to add any refrigerant. The biggest problem I have with the old gal is keeping the hot outside air out of the cab. If it wasn't for that this thing would cool awesome.