i cut out the old panel leaving a 5mm overlap then use a swaging(flange) tool on the edge on the new panel and punch holes around the edge of the existing panel, then weld through the holes to hold the new panel securely in place, then seam weld 1 inch leave a 2 inch gap, weld 1 inch etc.. repeat the weld-gap-weld until the whole seam is welded without putting too much heat in one spot and warping the panels. After welding i use a 10mm belt sander instead of a grinder to grind the welds, if the welding is good enough you can completely hide the fact that it has been welded even without covering with paint or primer, using the belt sander
