Which stud are you talking about? One of the two that are integrated/welded into the metal frame of the heater box, or the one that is clipped onto the outer edge and has the integrated ground strap?
If it's the latter, they reproduce it and you can find it on eBay. I had to replace mine with this one because I bent mine beyond accepting the nut along the entire thread:
https://www.ebay.com/itm/393981832146
The only problem with the repro one is that it doesn't include the ground strap, which is extremely important since it prevents the coolant from degrading the heater core via electrolysis. What I did was just purchased a generic groundstrap (roughly 5-6") and relocated the large eyelet to the bottom mounting screw for the heater box (the ones with the teethed washers that you access from inside the car).
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.
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1979 Trans Am W72 400/4-Speed WS6 - Starlight Black Hardtop