The link to the GMC truck Site just gives you a sign on page. However, the site is very easy to join. There is a forum on there called "Metal Shop" and Ray Stanley "Metalman" has a photo documentary of replacing those panels. I don't know if he has any patterns but if you catch him at the right time of year, he may be able to make you some. I believe that he is making some other panels for Jim Carter, as part of his work right now.
