Hi there, gtipping is correct you need to shrink flange. You can do this simply by using a pair of pliers and twistingto left or right and then move a little along flange and twist other way so you form a inverted V in flange. You will then need to beat down V back into itself working from bend to outer edge. This will shrink metal back and form curve, you will need to do afew along edge and maybe you will have to repeat but do it a bit at atime so you dont overdo it. To keep panel flat, lay piece on flat metal surface with flange up and hammer on edge of flange. There is a great DVD you can get by David Gardiner called Bodywork Restoration Tutorial this can be bought from
www.metalshapingzone.com.
hope this helps
tony