Before spending $50/lb for R12, consider a comprehensive leak inspection using small amount of refrigerant and pressurizd N2. A sensitve leak detector is used to check for leaks. On older cars, the expansion valve orings in the connections are prone to leak.

Suggest an overhaul of the system and convertering to R134a. That way your money is spent on something that is maintainable for the long term. A properly done R134a retrofit should deliver adequate cooling.

Overhaul/retrofit would require replacing all orings in connections, removing evaporator and replacing all orings in expansion valve and thermal bulb connectors, flushing lines, new R134a receiver/drier, retrofit fittings for R134a.

good luck