Unfortunately, the "nut" is a four-sided nut that is welded to the frame from the inside. I had one break loose and the only way to fix was to use a torch to cut an access hole in the side of the frame adjacent to the mounting hole. Then, weld another nut over the hole, then weld the access hole shut and grind to pretty it up.
It isn't very difficult, but you do need the torch, the welder and the skills to do the job. Otherwise, you'll have to pay someone else to do it.
To me, RC is kind of the Harbor Freight of springs. Sometimes you get a good deal and sometimes you don't.