You could just fill that old body damage with any plastic body filler,BUT if it was mine I would hammer & dolly the metal out a little better first. It just depends on how much filler you want in it & how much time you want to spend on it.It looks like someone stretched the metal pretty good by not properly working the metal out and hid the evil deed with body filler.

1957 Chevrolet 1/2-ton Stepside LB in the Gallery
My Photobucket shots

The person who has nothing for which he is willing to fight, nothing which is more important than his own personal safety, is a miserable creature and has no chance of being free unless made and kept so by the exertions of better men than himself." - John Stuart Mill (1806 - 1873)