Gotcha. Since you mention it I can see that's an issue. You gotta do something though. How far would it drop if you ground the head off the bolt? Far enough to get at it with the grinder at the nut side? Probably not..
As MB says, welding a piece of rod to the nut may give you some leverage; obviously you need to be careful not to weld the nut to the bolt in that tight space.
A good soaking with a penetrant, I've used Reducteur H-72 with some success on seized bolts.
Maybe just heating the nut with a propane torch will loosen it. The heating method worked with the body mount bolts on my GQ, those were rusted solid at the time I replaced them.