R12 is what my brother in the A/C business refers to as liquid gold. I have converted a couple cars to R134A and other than blowing the lines out with denatured alcohol, new o rings, new reciever dryer, different refrigerant oil, and a fitting change to pump in the newer refrigerant you will never notice a change in temps compared to R12. Some people say it's not as efficient. If that means R12 can get 2-5° colder at Max cool then who cares.
I would recommend getting an A/C shop to swage new rubber lines on your old hose metal ends, because that old system probably leaks everywhere. Remember those hose lines are over 50 years old and in a dry climate.
We had the A/C lines redone on my dads lil red express. They had to cut and braze barbed fittings on the ends of the metal lines to accept the hose end crimp sleeves. Used new rubber composition Goodyear Galaxy hose. They also rebuilt the compressor.
It held full vacuum when we pumped it down and shut off the vacuum pump. My brother pumped it full of R12. You need to vac system down for an hour or 2 so the moisture in the system boils off and the pump removes it.
