The factory nuts are enclosed in a metal cage, or have wings (ears) on them and the cage or ear is welded to the frame rail.
I believe he is proposing that you drill a small hole in the area where the cage or ear should be and then weld thru the frame rail and attach the cage with that weld., then brush the weld flat and done. No structual damage to the frame.
The hard part will be positioning the nut for welding..,since it is probably laying somewhere in the frame rail ....
Another suggestion is to drill a hole with a hole saw from the opposite side of the frame rail and insert and weld from the inside, then use the slug and weld close the hole.