1969 Dodge Dart GTS: How to remove rear exhaust hanger to replace

That's not factory, just in case you did not know but, if it were me, I'd cut the nut off with a cutoff wheel, grinder or air saw. It's not any special bolt that needs to be saved. After I cut the nut off, I'd remove the bolt head with a magnet or needle nose pliers thru the frame hole, unless you have an extra 1/2" combination wrench laying around that you could heat & bent to go into the hole to hold the bolt.