I got these shown below from ebay years ago in case I got another Pantera to build along with the wifes divorce papers. My cars have them welded in the area behind the rear window. They have a slight curve to them, and measurements are shown in case maybe you could make some. I hear reinforcements should also go near the corner windshield and around the tail lights; although others here undoubtedly would know more about where are these areas are and how to reinforce them.
MetalReinforcements by JanDaMan, on Flickr