Get a set of step pliers and flange the entire length. This makes about a 1/2" flange the same thickness as your replacement metal. Hang the panel with Cleko pins and spot weld beginning in the center of the panel and go back and forth every 12 inches to avoid building up heat. Go over it time after time until you have full coverage. Remove Clekos, weld the holes shut and grind until smooth. Very little filler will be required.
Good luck, Dave
Edit comment - heat is you enemy - be careful!!