TL/DR summary: Help, my '76 Charger's heater core is a colander and parts availability is non-existent.

Last fall, my Charger's heater core popped while driving to LS Fest East. Up to that point, the HVAC had been working fine aside of the A/C system, which has a hose taped over with Gorilla tape. Having hot ethylene glycol steam pouring from the dash and vents was not a great thing. Bypassed the heater to finish out the season, removed the HVAC unit from the car, cleaned the mouse house out from behind the evaporator, had the heater core re-cored by a reputable shop, reinstalled. Heater worked awesome...for fifteen or so minutes, then the steam appeared again. I'm tearing the dash out of the car in preparation for another HVAC job, and I'm looking at options. There hasn't been an A/C-equipped heater core in the parts system since about 2017, per my investigation. Chances are good that I'll still be dealing with the same rot qualities if I get lucky enough to junkyard one. I think there is a design difference between 1974 and 1975's heater core.

I did come up with a possible option: an E-body HVAC case. They're available. And to my understanding, the 1971-up B-body's firewall was the same as the E-body. As long as the vacuum lines are hooked up properly, what would be the issue?

I'm not the expert here. I'm just looking for a way to improve the drivability of my car. I want heat in the cooler months without the coolant sauna and air conditioning in the summer months so I don't sweat-stain the white pleather seats. And I do not want to put an add-on style A/C system into this car.

You can see the pictures of the original teardown here (mind the PG-13 language): BangShift.com Post: The Charger - The Heater Core Job We Should Have Been Worried About