No need to over examine all the potential possibilities. You have been given the correct answers already... your origianl post said just about everything you need to know. Your rigs runs OK with no a/c... when you add just an extra bit of heat, which is what the a/c does, the engine temp rises dangerously.
This is a radiator problem. If you feel like trying to get it cleaned or rodded... I wish you the best of luck. I just spent a year trying to keep my old rad around by doing just that and ended up wasting time but with semi-acceptable results until the truly hot wx hit. It is a very old piece of equipment. You can go round and round trying to save a penny or a minute. honestly, just pull the trigger so you'll have new shit keeping your engine running safely.
REPLACE the radiator with a new one and the fan clutch too. Then you'll be free to drive the CA 120 with A/C.
This is a radiator problem. If you feel like trying to get it cleaned or rodded... I wish you the best of luck. I just spent a year trying to keep my old rad around by doing just that and ended up wasting time but with semi-acceptable results until the truly hot wx hit. It is a very old piece of equipment. You can go round and round trying to save a penny or a minute. honestly, just pull the trigger so you'll have new shit keeping your engine running safely.
REPLACE the radiator with a new one and the fan clutch too. Then you'll be free to drive the CA 120 with A/C.