Are you going to put a vinyl top back on it? If so, the repair would be much more forgiving with respect to bodyworking the weld seams. Does the rust damage go any further back? If not, it seems you could patch it with a good front section from another car. Ideally you could find a good entire roof skin off of a donor car that had a good roof but other problems. A whole skin would give you the best repair. Are you going to do the welding?