I've been using my Miller 130XP to weld patches into my Dart and I'm finding that a MIG is really not the greatest thing since sliced bread for this. No matter how slow I go, keeping the heat down with single spot welds, the patch area always comes out warped and requiring bondo. The actual weld bead is the worst. It always ends up sitting lower than the surrounding metal so that even when I grind flush it's visible as a bead. I've never used a TIG but I've heard it's much better for this stuff.

As for getting the curvature, you just have to beat it to shape unless you have some kind of metal roller. The old Ferraris were all made with just hammers and tree stumps. I have a big wooden mallet I made for this purpose. The head is the same shape as the safety cap for a big industrial gas (nitrogen, argon, ...) tank. A few whacks with that along the center of a piece of metal--even just using the garage floor as a backing--will get it curving.