If you don't know how to weld then you could
1) remove old nuts with hole saw
2) lift body off frame
3) drill holes in the top of the frame and drop in long bolts
4) insert crush sleeves from the bottom (make them exactly the right length, flush wit bottom of frame, and make sure they fit snug in the hole saw holes)
5) bolt up with nuts
The welding fixes are better though, so please consider them first.