hello,
if I'm going to go with 134a do I need a parallel a/c evaporator?
https://www.ebay.com/itm/1968-72-CHEVELLE-MONTE-CARLO-EL-CAMINO-A-C-EVAPORATOR-COIL-AC-AIR-CONDITIONING/400282684072
flush the system remove all oil. i use Alcohol but you can find other products to use. Rob also had a pretty good write up on rebuilding his system and i believe he went r134 also.