Bracing the doors won't hurt. You have to brace them on a convertible. You should be fine if you replace one panel at a time. It is the best way to do it for panel allignment. When you cut off everything at once, the car will become unstable. Also, make sure you are on a level surface and the car is level before welding it back together. The old rule of measure twice, weld once, is a good one to follow. Before you do a final weld on the rear quarters, mount the rear valance, etc. You may know all this. I don't know your level of experience.
