Other than that try a local body supply shop or ebay for rocker panel replacement. Wafer wheel and jigsaw the panels out. Use panel epoxy to glue them back in. Your talking $300 to $400 but the problem is that you will find even more once you start looking.
We happened by the Brothers compound recently to see the collection of pickups. While there, we were also lucky enough to witness a rocker panel replacement on a '67-72 C10 firsthand, which we obviously didn't hesitate to capture on film for your viewing pleasure. (Please keep in mind that while Brothers offers the products to restore your truck, they're not in the business of installing the parts they sell. This is a company-owned vehicle used to test products.)