Mmm. There's a small problem though, close quarters. There's hardly anyroom for anything. It would be near impossible for me to angle grind it...

 

Just a thought... What if you drilled a hole in the top and screwed an easy out into it to hold it while you loosen the nut?

 

or just tackweld whatever is not supposed to move?

 

What about cutting a groove in the top of the bolt head so you could hold it with a big screwdriver or something?
Or even cut the sides off the bolt head and then you could get visegrips on it?
A sawzall would work, with a metal blade. (I love my sawzall!)

 

You could use a cutoff wheel to cut a screwdriver slot in the top. Then get a similar grade 5 bolt and square off the head, and dog point if you can. Then weld the replacement in place.

 

Here's another thought. Try wedging a screwdriver between the rail and the floor pan. Pry up on the rail while turning the nut.

 

As a last resort. Go to the local autoparts store and get a tool called a "nut splitter". It will hook around the nut itself and as you tighten down a bolt on it, a chisel will be forced onto the nut and basically crack it in half. Check out some pictures online and see if it will fit in the small areas first.
Personally, I would use a dremel tool and a cutoff wheel to cut any end of it off. (or put a new head on it the bolt/nut) Use the flex shaft and a grinding wheel on it to get into REALLY small areas that no other tool could reach.
Don't have one? Any excuse to get a new tool is a good one. I have 4 separate ones and use them at least 3-4 times a week in the shop and all over the house. Get a cheap knock-off at harbor freight for $10-$15.

 

cut it. could probably get one of those small hand held saws in there. or you might be able to glue the bolt to something so it does not move.

 

I like this idea. That way you can just reuse the bolt and never have to worry about it spinning... Unless your tackwelds break

 

I wish i could weld it, but:

1. I dont have a welder
&
2. I think I would set my upholstry on fire by doing that