Adding AC to a non-AC 73 Hornet

I know that in the Hornet and Gremlin of that vintage, the big difference is the center HVAC distribution box. The fan housing on the right hand side is similar, with a couple minor differences, but the A/C cars do NOT get an underdash vent on the right side.   The cables are different, the controls are a little different, and the heater control is different.   The '73 and 4 cars have a full flow heater, where the A/C cars have a valve that's vacuum controlled.   When the blend door to the heater section closes, it closes a vacuum switch at the blend door, which closes the heater valve under hood so that it doesn't bleed heat into the A/C system. I have one of each system in each of our Hornets, and there are many differences.

IMHO, best bet would be to grab the HVAC setup from an A/C car (comes out in two pieces) along with the dash controls. Trying to piece and part one would be challenging. Going with an under dash unit might not be a bad idea.

There's a place in Texas that makes the complete R134a replacement for everything under the hood back to the expansion valve inside the car so that you can maintain factory A/C function in an A/C car, and they have solutions for non-A/C cars also.

There's replacement foam kits available for either factory A/C or non A/C cars, so you can rebuild the whole factory HVAC setup if you want.   That's what we did.

Here's the kicker that I didn't really think about when I did the A/C car.   The area where the radio goes is SHALLOW with the A/C setup.   Factory radio territory here, you won't get a CD player into it.   We ended up using a modern USB/Bluetooth head unit that has a million colors available, etc. Half depth unit.   Auto sound makes a modern head unit with the knob controls to look more vintage and it will fit also. Cassette units would be tight in the factory dash with the factory A/C.   The case just takes up that much space.