All of the light-duty, '60-'66 Chevy/GMC trucks, Panels & Suburbans, had an inner rocker panel. Without it, the outer rocker panel is structurally unable to retain its shape. Perhaps they were removed or damaged, via rust, so they no longer exist. The outer rocker panel is quite flimsy with the inner rocker panel, in place, and spot-welded to the outer rocker panel, along the top & bottom, and captured at the lower ends "A" & "B" pillars. Since they are not currently there, it should be a piece-of-cake, to install replacements. During the process, you may find some remnants of the original inner rocker panels or evidence, that they once existed.