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Buy a crappy 6 point socket set and select one size smaller socket and pound it on with a hammer to remove the bolt. Could be that a 12 point socket may be easier to find in the crappy sets. If you go to a place like Tractor Supply Company (TSC), you can buy the sockets individually for a low price. Just don't get too attached to them.
and if that doesn't work, just grind the head off with your cutoff wheel/grinder and once the girdle is off, you should be able to grab the shaft of the bolt with a pliers and loosen it right up. its not rusted in there, its just torqued pretty tight.