I bought a '78 shortbed stepside for a K10 restoration I have been working on for 8 years. The flat portions of the box are very rusted and I am weighing my options for fixing/replacing. Replacing seems the best route to go but locally, I can only seem to find reproduction panels for a 1966 and older. Can someone tell me what the difference is from 60-66, 67-72, 73-76( shows this year break) and 77-87?