I have had no luck coverting older cars to R134. It is molecularly smaller than R12 and therefore finds seals, and even hoses it can leak through. Short of changing out everything there is an alternative. HC 12A refrigerant is molecularly larger than 134 or regular R12. It also takes less to achieve the same cooling capacity. I have brought 3 AC systems back to life on one 6 can order from Foxtools in Nashville, TN.
foxtoolsupplyThe downside is that no shop will do this for you. You would need the tools to do it yourself, and to do it properly you shouldn't just purge the old stuff into the atmosphere. HC12A reportedly does not deplete the ozone, but it is not something the government of most states approve. It works great though.