Thats a tough one. You need to get inside the area where the nut is, basically each 3/8" bolt goes thru a j-nut. the original ones actually have a nut welded to the j-nut clip, the aftermarket repros are a one piece stamped j-nut with no separate nut.
You may have a couple where the nut has separated from the body of the j-nut clip, you may be able to keep those from turning with the tip of a flat head screwdriver thru the slot where the j-nut is installed.
For the stripped ones, sticking a wedge between the bracket and the floorboard may allow you to back each bolt out.
If all else fails you may be able to cut the bolt heads off with a cut-off wheel or grinding the bolt heads down, or even drilling the bolt heads off.
I'm pretty sure they're 3/8" diameter bolts.
Good luck.