Problems getting bolt out of exhaust hangar bracket

[bull]

One of the nut zerts (for lack of a better term) has broken loose from the frame rail where the driver's side exhaust hanger/bumper guard bracket bolts in place. I don't know what to call the thing but if you've ever taken those brackets off (as seen below) it looks as if the frame rail itself is threaded but it's actually a threaded insert. Well the sucker broke loose with the bolt still in it and the bracket still hanging on. Any idea how to hold a rounded off nut while trying to remove a rusty bolt with only about 1/4" inch of space to get to it?

[UFO]

I assume you want to hear a better plan than cutting open the trunk floor above it.
Getting it off ,you can cut/grind the head off.Then pray that the pcs inside the rail can be worked out another hole.
After it's out of the way,I'm thinking a "U" shaped pce with a nut welded on it to slide up into the rail then weld the sides to hold it.

[bull]

I thought of cutting a hole in the side of the frame rail, actually. It would be much less obvious once repaired since it could be ground down evenly, painted and then covered with undercoating.

Taking the head off is another idea, and probably the best one, but then there's the bitch of finding a 7/16" captive washer fender bolt to match the other three (if that's something I decide is going to bother me).