I've got a broken stud that I'm gearing up to replace, either once spring comes around or once my dad's hot rod is together enough to give up its spot in the shop for a while. I can see mine just by looking at the manifold, there's one hole that just plain doesn't have a stud sticking out of it. They can be kinda hard to see around the heat shields, though.
My dad had the same problem on one of his WDs and told the mechanic to cut a hole in the fender wall to get to the studs, then put a rubber bung in it when he was done. Apparently it saved some time. I'm hoping to get mine from above with a cheap 90° drill adapter and some left-handed bits, though if I can get to the stud with the welder, I'll be trying the weld-a-nut-to-it trick first for sure. (Thanks for the writeup Snowboard!)