It it were me, unless restoring to factory specs, I'd replace everything with a modern set up and pull a vacuum and be done with it. R134a just does not cool nearly as well as the original R12 Freon. R134a will not only leak from the 0-rings if not replaced, but can even leak through intact hose OE walls because R134a molecular structure is smaller than Freon.
That would be an expensive and time consuming proposition. I might consider this if I were building a daily driver but I am building an occasional use truck. My 1987 GMC is at the body shop and that A/C hasn't worked in a long time. The truck was not really used much for the last 20 years. Now that I am restoring it, I plan to get the A/C working again.
My plan is to replace the compressor. My original one is really no good, noisy. I will replace the condenser, evaporator and accumulator since I have them off the truck now. I will replace the hoses with barrier hoses, again since I have them off the truck. Then I will charge with R134a.
My personal experience is R134a does a good job replacing R12. I live in Connecticut and we do get some really hot and humid summers but nothing like FL, TX or AZ experiences, especially with the sun load in those states. The systems in these trucks is way oversized. A Suburban has the same displacement as a regular cab pickup despite the volume of the cabin and amount of glass.
I would also suggest adding some insulation. It will cut down on the heat load and make the cabin quieter.
