I cut a section from another Cutlass when I fixed mine. That's about the only way to do it. If it is really bad, you may want to swap the whole roof. It may be easier to do this than repair it in the first place. I wish I had, because mine came from a batch of cars which did not have it's parts primed before welding whereas other Cutlasses were built better with full coverage of all their parts inside and out.