As for the R134A refrigerant, it will work fine in an older system. However, you will likely need to change out the oil in the compressor to an oil compatable with the 134. Some oils were compatable with both, but I wouldn't take the chance. They used to make R134-R22 adaptors to go on R22 charge-discharge fittings. 134 isn't as efficient as the R22, so you will have less cooling capacity than before. Usually the biggest difference in the two systems is the size of the evaporator (the finned part under the dash). Depending upon how good the original systems were, you may never notice a difference. I can't say about the trucks, but the ACs on Chrysler cars of the same era would let you hang meat in them if you needed to.
My truck had a hang on system installed when it was new, but if I had a parts truck with the complete system, I'd change it out. Just my preference though.