R4 compressor is the only direct fit for your bracket setup. There are some Sanden conversion kits.
BTW R4's were continued for use once refrigerant was switched to R134a; the 1994 and 1995 model trucks still used the R4. The difference between those trucks and earlier is generally the condenser design and the orifice tube design, as well as slightly different charge amounts.
There are a few different places to buy from if you want to do the Sanden conversion; some offer complete kits ($$$) others as simple as just the bracket and you source your own compressor. Search keywords like "r4 sanden conversion" - the Sanden is definitely a better compressor design than the R4, just beware though that Sandens knockoffs are produced in huge numbers by the same Chinese jokers that are selling R4 junk. So just because it's a "Sanden" doesn't mean it's top notch.
Look closely at what you're buying.
Richard