I'M NO EXPERT, but i talked to a friend about this same thing for my '67. he is part owner of a repair shop and worked for a dealership for a number of years and does a/c work. this is what he said:
The problem with the older a/c systems are the materials used. The hoses and seals for a r134a system are chemically different than ours. while you could do a flush and refill, you will find that you will have to refill the system more often. On a molecular level, 134a refrigerant is smaller than r12. as a result there is more leakage through the rubber hoses and seals. To prevent this you would have to rebuild the system with new materials.
He also cautioned about the r12 direct replacements. (I'm NOT talking about 134A!!!) There have been cases where explosions have occurred because of the flammability of the product. An a/c shop has no way of knowing if you have R12 or one of the flammable r12 replacements when they evacuate a system. As a result he cautioned buying any large R12 bottles (like a bbq grill size) as it could be used refrigerant. As a result I am always looking for unused one pound cans of R12 at garage sales and I have decided to keep the factory system and keep using R12, but only from the one pound bottles.