replacing full rear quarter panels dart sport

Watch the Craig Hopkins video. He is an AMD dealer/ approved installation center. He does a set of panels on a 68 Dart. Very informative. You can Google it by searching "craig hopkins panel replacement." Buy the DVD it's worth it.
Melt the lead filler with a blow torch and use a wire brush by hand. You only want to get the lead hot enough to melt and keep moving the heat. You shouldn't have to get the sheet metal that hot to warp it. If you do just reheat it and cool it rapidly with a wet rag to shrink it back to shape.