86 740 AC Changeover?

I have to replace the compressor on my 86 740. After 300,000 miles, my AC man tells me it's giving up the ghost. He says replace the compressor/clutch and receiver/dryer and he'll recharge with R12. But at $60+ per pound I'm wondering if 134 is the way to go. Mr. AC says since there are no system leaks, and it cools fine, keep the same refrigerant. Comments, please.