I also have metal man rear corners. I pretty much rebuilt the interior sections including where the rear step fastened on. I made templates of bolt holes and used floating nut carriers. Used a thicker gauge steel on part of the fender well too.
I treated all the interior with epoxy filler and primer.
The front lip of the new corner is supposed to be sectioned in to the fender well material with welding. You can do it the other way by laying in on top but the fender where the welting goes might not be a good fit. I have not done that part yet.
Welding in the corner should be a little at a time so you do not warp anything. Start by a spot weld in the middle of the corner and bend to shape.