68 Cuda grill studs............ Any tips ?

I could be wrong but I think the original studs had 10-32 thread so you would need 10-32 nuts for those. If your new studs are 10-24 thread you'll need 10-24 nuts. Any hardware store should have them.
I wouldn't replace only those broken studs and have mixed sizes though. I replace every stud so the same 10-24 nut fit all of them.