Help! Foolish me playing mechanically inclined again.

Heat may work. If not, the dremel tool, with the cutoff wheel, cutting the nut will work wonders. If you can't make more than one cut, because of lack of room, make the cut facing you and hit the groove with a cold chisel.

If a else fails, while you might not be able to get a sawzall blade in there to cut the bolt horizontally at the base of the nut, you could cut down the middle of the bolt and through the nut.