My bed sides do measure correctly (79 5/8"), so that's not the problem; the front bed panel is indeed too far from the back of the cab as mentioned in previous posts.
The front bed panel repair by the previous owner may be the problem. The mounting of the front bed sill is pretty well fixed by the existing holes in the frame, which do not appear to have been relocated by the PO. Here are some better pix of mine:
L side front sill 1 L side front sill 2 (these 2 pix display as cropped, not sure why)
And here is a picture of a bed off the web--clearly shows the panel mounted to the front of the sill, but the resolution does not permit viewing of much detail:
bed assy web pic Can anyone post pictures of a correctly installed front bed panel, specifically the bottom?
Thanks,
Warren