If the bracket itself is still attached to the back of the seat, you can drill the cross threaded holes slightly larger and then use JB weld or marine epoxy to secure another new pair of regular nuts to the back of the ones already on the bracket. Then just get a pair of slightly larger bolts.
All the JB needs to do is hold the nuts in place so you can get the bolts into them. The bracket itself would still be secure and, when bolted, would be holding the seat against the seat mount. (I mean, theoretically the nuts could be loose, but you'd never be able to "hold" them and thread your bolts through the toolbox/seat mount.)
MF Battery, Iridium Plugs, RR relocated, Voltmeter, 170/80/15 Kenda Kruz rear tire, DIY Samsonite hard bags, DIY shaved seat with Beaded seat pad ('cause that's how I roll) and the dreaded STATOR CHANGE.
Shining Black Bess '86 VN750, retired for parts after a fried stator and being knocked (kee-runch) in her parking spot.