A NEW GM box end is rounding the head. No room for socket. No room for grinder except mini die grinder. sawzall stroke would be too long with limited clearance.
Air chisel? I'm an artist with these. Acetylene torch?
The only way I see is to waste the bolt head since the nut is welded on the back side of the brace.
BTW I can't find the die grinder or the air chisel.
Grind out a 9/16 12 point to fit better?