[i4abuygto]

Which stud are you talking about? One of the two that are integrated/welded into the metal frame of the heater box

It is the lower one that is integrated into the heater box - I haven't removed the box yet so I don't know exactly what is there to work with on the broken stud. I know the stud broke right at the base of the nut which means I should have some bolt there to work with.
I thought about just welding a good 1/4" threaded stud to the broken stud. Or, maybe JB weld it and rethread it.
These studs must not take much to tighten- I hardly put any torque on it when I was tightning it down.

As for the ones that are part of the metal frame, there is a guy on YouTube that goes over replacing the heater core for his 1980 (or 1981, not sure) and he runs into the same issue and describes what he did.

Thanks, I will try to find this YouTube video.