If there is enough thread too put vice grips on.
I think your only hope is to pull down hard enough on the bolt that it unscrews from the nut then 2 people holding the hood and titling back and forth to work the nut out of one of the holes in the brace. Remember the old wooden toys that had a BB to get through a maze.
They make captive nuts that go on from the outside but I cant picture what you actually need.
Ugh? That's Biatch?
Which one is it? Front or rear? Either one, you might be able to get away with drilling an extra hole, about 1" diameter and see if you can somehow fish a nut with a star washer in there to catch the bolt. This method might compromise the structure though.
Otherwise you'll have to cut the old one out and weld a new one in.
Either way, it's going to require some surgery.
… Still trying to think of a way to get a clip-nut in there...
Are you sure there are J clips on the hood ?Pretty simple fix. Protect the car with blankets or something, take a hand grinder with a cut off wheel, carefully cut the head of the offending bolt off. Remove remaining 3 bolts, remove hood. Remainder of bolt will be in what’s left of the J clip, fish that out, remove broken J clip. Install new J clip, reinstall hood with new proper 5/16 bolt. Drink beer.