If you are sure the 2nd fender with the good lower section is not rusted inside, I would cut the skin off that and patch it in to the other fender, leaving the bracing. Do like pic #2, except cut it right at the edges where it folds around. Easier to weld there, less warpage. Also, go further under the bottom.
Might be easier and better just to buy a patch panel.....they are not that much