What needs to be done depends on the current condition of the system and how long it's been out of commission. A one size fits all, step by step procedure would be nearly impossible.
Here's the basics:
-Visually inspect all of the components for damage.
-Remove the lines, hoses, condenser, Suction Throttling valve and Expansion valve. and check for internal corrosion.
-Pressure test the evaporator.
-Test the STV and Expansion Valve.
-Drain and oil flush the compressor. Add back 10 oz of oil. Replace the compressor shaft seal.
-Solvent flush the heat exchangers and lines.
-Reassemble the system replacing any seals, o-rings, and a newreceiver/dryer.
-Evacuate and charge with refrigerant.
If the major components are ok, replacement only involves a dryer, oil and some seals.
- I would suggest removing the evaporator to test it and clean the case. There will be 45 years worth of crap in the case and those evaporators are prone to corrosion.
- I would also suggest having the STV rebuilt and calibrated for R134a if you plan to convert the system.
-It would also be a very good idea to replace the 45 year old rubber in the lines. They can be rebuilt with modern barrier hose rather inexpensively.