Classic Auto Air's "Original Air" division has "underhood upgrade" kits- everything from the firewall forward. If all your interior components are intact (heater/air box & evapaorator, vents, controls) then this will get you all you need; with all new, modern components.
The bad part: only currently available for B bodies. But give them a call- they offer A body parts too, and may be able to piece together a kit for you. I wonder how much is actually different under the hood between A and B bodies anyway... a couple of hard lines with different bends?
Use the "Contact Us" icon on the left side of their page and find out:
MOPAR - STAGE 2 & 3 | Original Air Group
73-74 Charger/Satellite A/C Performance Upgrade Kit Small Block STAGE-2 | Original Air Group
