Let me go pull a stud from a '68 grille I have, and I'll give you the dimensions. It may be "all thread" with a single pitch/diameter, or two different pitch and diameter. But it will help you find a source/solution. I'll post back tomorrow.
The larger side of the stud (3/8-14 UNS HSS) is into the grill. The smaller side (5/16-18 UNC) is the retaining nut side. If you have taps, check the grill to be certain the threaded bosses are correct. You could use a piece of "all thread" that fits the grill, and then the same nut size to secure it. That may be the easiest route to use.
Thank you both. I had to buy a new re-pop grill so I'll check the thread to be sure. All thread sounds like a good idea.