Take your time and DON'T force it. My buddy and I were changing my front struts and we were experiencing the same issue as you. But on my side I was slowly working it back and forth and hitting it with PB blaster. My buddy thought it would be a good idea to use an impact wrench on his side of my truck. I, of course, let him and the nut welded in the inside of the frame snapped loose.
I had to take the truck to a mechanic friend of mind who used a torch to cut a window into the frame and re-weld a new nut in place. He then welded a patch over it and painted it over. All this just to get the bolt out so I could replace the bushings like you.
Just go slow and when it gets tough to turn lube it up and tighten it back up a few turns. Then start loosening it again. It should come out.