So it's most likely stripped...since the bolt just keeps turning, but not moving? That's great news 
That would be my guess unfortunately. I know it sounds dumb, but hopefully you were loosening it (turning counterclockwise) the whole time? At this point it doesn't really matter, if you can spin it 360* a few times and nothing happens it's stripped. You might try using an impact wrench, sometimes the threads will bite in. With it cranked all the way in like that there will be pretty significant downward pressure on the bolt, so, cutting it would be sketchy because the suspension is loaded. Heating the bolt and plate might help it grab. Regardless, you should be careful around that thing because if it decides to let go in a hurry parts could go flying.
Maybe someone has a better idea, everything I can think of doing to get around that issue involve various degrees of "hold my beer and watch this" kinds of results because the suspension is loaded.
