Flush the hell out of anything you don't replace.
Put a little oil (Mineral oil for R12, PAG or POE oil for R134) in the accumulator to soak that sock, I also put a little in the evaporator and condenser, and just a few drops in the compressor and worked that around by manually turning the compressor.
If using R134, about 75-80% maximum of the amount of R12 you would normally use.
Use the green 134 O-rings/seals where applicable if using R134.
Once everything is bolted together, vacuum suck the crap out of it and make sure it holds for a long time. Don't just suck it down and call it a day. You want to flash off every bit of moisture and dry the system. Moisture and R134 = hydrofluoric acid. Probably not even close to being good for your system.
You don't have to, but you may want to think about putting in a variable orifice for R134. Again, not necessary, but the variable one can adjust for the pressure changes (higher pressures with 134 over R12.
Go slow and take your time. Ensure every leak is chased down and eliminated. Don't overfill the system. DON'T try to add straight liquid refrigerant if the engine is running and system is operating. If the system cycles with part throttle, you probably haven't added enough.
Best to weigh the cylinder and add the calculated amount. You don't absolutely have to, but if not, use a thermometer in the vents to help get your air chilling at a/c idle speeds.