Aside from the flushing, swapping o-rings, reciever drier, hoses, removing some of the old internal parts and possibly some other changes, I understand that changing the condensor to one of the new design is what really enhances the cooling performance of the r134.

1970 Plymouth 'Cuda #'s 440-6(block in storage)currently 493" 6 pack, Shaker, 5 speed Passon, 4.10's
1968 Plymouth Barracuda Convertible 408 Magnum EFI with 4 speed automatic overdrive, 3800 stall lock-up converter and 4.30's (closest thing to an automatic 5 speed going)