R12 to R134a worth it?

See here, here, here, and here for the info you need to do an optimal-working R134a conversions on an old Mopar. If you're working with a '74 or newer A-body factory A/C system, remove the EPR valve and install a clutch cycling switch (also applies to '62-up bigger cars, but A-body factory A/C had a clutch cycling switch and no EPR valve thru '73 -- the dealer/aftermarket "hang under the dash" systems have clutch cycling switch and no EPR regardless of year).

To answer a related question: Use R12, or do a proper conversion and use R134a. Don't use anything else. Any of the hydrocarbon (isobutane/isopropane) "refrigerants" -- "HC-12a", "RedTek", "OZ-12", etc. -- is a very seriously foolish gamble, and the "Colonel's Secret Blend of 13 Herbs and Spices" blends like Freeze-12, FRIGC, etc. may work in the short term, but their various ingredients leak at different rates, changing the composition of what's in the system so you can't just top up and have it work well...and most shops will not touch a system charged with something other than a legitimate refrigerant (12 or 134a).