[argonaut]

Wow, I have never seen rear sill tabs on the frame like that before. Is your truck a 60-62 by chance? The older 2wd frames were wider and thus so were the mounting positions on the bed rails. But maybe all the 4wd frames were narrower like the later model 2wd frames.

When I put a 62 bed onto my 66 k10 instead of replacing the rear sill I just drilled out the spot welds on sill mounts and tack welded them into the narrower position to sit directly on the frame rail, like thr 63-66 pickups.