If you use something other than mineral, use POE. It bridges R12 and R134a. Not sure about all the newer fancy refrigerants. Hell there were some guys using cans of duster or something similar to charge their systems, it might have been r1234a or something inside.
I would just re- oring everything, orifice tube (which can be a pain in the ass) assuming not Txv (90% sure I dont think these trucks use txv but it's been a while), and dryer swap. Flush the old oil, new hoses wouldnt hurt. New high n low caps. Reuse condenser, evap, compressor. Add a fan to the condenser for even better performance. Oil n r134a or whatever you choose. You'll be icy cold in no time with a system that'll last unless you dont run it and the rings go brittle. My 2 cents.
Harbor garbage has gauge sets that are ok, and vacuum pumps that work by venturi. It's better than nothing, I've seen lots of junkyard repairs that do ok too where guys will skip it all and just charge it up, I dont recommend that though. If you yet bored you can make your own vacuum pump from an old fridge compressor and some brazing. Just gotta oil it up. Atmosphere in the lines is your enemy, it'll turn the system acidic and kill your compressor. The dryer will prevent that some but if it's a new charge I'd swap it for peace of mind. If a job is worth doing, it's worth doing right. You could probably charge it up and be done and go for years if an oring hasn't cracked, or you could have a snapped off orifice tube and get crappy performance and have wasted all that time if you skip steps.
Not sure what you meant by ATF?
Either way, add a shot of dye, it'll make your life easier WHEN, not IF you have a leak if you dont have a sniffer. Add an electric fan that switches on with compressor and you'll freeze your nuts off with r12 or r134a or whatever else you choose.