It is sometimes easier for a beginner to add, or reduce the shape of an existing welded in panel, than to make a panel from scratch that fits perfectly. To add shape, you can stretch with a ballpeen hammer, then smooth with a hammer and dolly. To reduce shape, you can use a shrinking disc or a torch. Since the existing panel is restrained, the edges cannot go wild on you... one of the hard to control things when you start out.
John
www.ghiaspecialties.com