The EPR valve (and the stock expansion valve) isn't calibrated for the pressures associated R134a, so you will at best lose efficiency.

The flip side is that the RV2 isn't really designed for the constant cycling it would see if you removed the EPR and went with a thermostat. Premature clutch failure is a very real possibility.

The RV2's are horribly inefficient (not to mention ugly and heavy), and the re-manufactured units are a crap shoot quality wise. The higher pressure require by R134a can blow out the seals and gaskets on one of marginal quality. I highly recommend getting a Sanden compressor (deals can be had on Ebay) and some brackets & hoses from Bouchillon.

Or stick with R-12 as Ma Mopar intended. Last check, price has come way down. I've still got a large stash in my garage.