I've found, as Gilles and others have, that the best way is to remove the backing plate by drilling the spot welds and straightening the parts separately. I gas weld the sheet metal itself but prefer brazing to join some parts together. A common problem is straightening the lower edge of the hood. Most seem to bulge out and resist straightening everywhere at the same time.
This was a first for me but I added a piece of bar stock to the inside. All the difference in the world!!!
The edge became stiff enough to stay straight once it's there, and the work doesn't show. I expect it to become common practice for me.
Bill, You could use the "Weld through" primer on both parts before welding them together. That might inhibit the rust.
