Yes I knew the green was the donor. I could see the red was part of the original truck. You say the red "was" on the truck. That must mean you already removed it? Another option is to try to fix the original while off the truck and weld it back on. That would be your best re-fit if you are drilling the spot welds out.
If you make the new roll under section, you can get the radius from using the other side as a template if both the orig and donor are gone.
Go for it. You didn't ask for advice on repair but if you do hammer and dolly, I have learned not too hit very hard at any time. Hit easy, easy, easy, slowly taking out the dent/crease. I have a carpet covered table with padding under the carpet and that is a good place to work dents. If I need less give, I put a piece of wood on top of the table and hammer on that as a backer. I have used soft grass areas also to work the major portion of a dent back to shape with a rubber mallet. Under a real "shade tree". If I am trying to take out a dent in a perfectly flat piece, I use a sold back but on a curved piece that would tend to give a flat spot. IMO.
