Can you feel if there are threads on the top side of the stud?

If there are no threads, then grind it flat, drill it out and tap it to thread it.
That or drill it out and use a bolt and nut (which is what most cast iron manifolds use).

The stud may have been welded in place on the factory headers like that.

Edit:
After doing an online google search of pictures of factory SRT headers I see some with no studs in the collector end making me thing that they are removable, as in threaded in.

Grind, drill, tap is your best option.