YOu need to cut a hole into the spring support bracket (which is welded to the "frame") to gain access to the now loose welded nut. You then remove the nut and the stub of the old bolt.
You can either weld in a new nut through your access hole or use a conventional nut. After everything is assembled, you need to close up the access hole you cut.
The neat way is to cut the hole from the top, through the floorpan. The replacement cover is attached with sheet metal screws and some sealant.
If you go from below, you must weld up the access hole you cut, so as to keep road grime out of the hole, where it would promote serious rust.
1992 XJ -- stock (RIP)
2000 XJ -- lots of basic mods. Sold to fellow forum member. I miss it.
2014 JK -- the project is just starting!