Looks like the other one was replaced at one time.
See if you can get that stud out; you can put a wrench on the bigger part closer to the manifold. Use some PB, heat, whatever, to break it loose. Worst case it will break off.
Or, back to your original question: what about grinding down the threads near the end, making it a nearly smooth stud part of the way, and then use a die to cut threads and use a smaller nut? That won't be as strong as a full-diameter stud, but I suppose it could work.
I'd try to get the stud out first.