A few choices:

1. Cut away enough of the body of the car to grab the nut with pliers (around the slot where it slides into the car).

2. Grind the head off the bolt so you can pull the bracket off, then cut the bolt with a sawzall through the hole right under the nut. The bolts cost nearly $100 and may be NLA, so make sure you have a replacement before doing that.

3. Take a flat steel bar that will fit into the hole and make a hole in it that will fit around the nut and hold it.


Also, when you put it back together, don't use anything but the factory nuts, because if you use the ones that come with the guide rod mount kits (Lemforder, etc) you will have the exact same problem as soon as you try to tighten the bolt. But, when I just did mine, there was only one set of the factory nuts left in the whole country after I bought them for my car. So, have fun I guess.