you just have to force the bed rails apart a bit. l had to do that to my truck when i restored it - this is not uncommon. loosen the fasteners that hold the sides to the sill, crossmember at the rear, and force the outsides apart. Then retighten. It may spring back so perhaps you have to push them apart a bit farther than the spacing you want to end up with. You don't have to move it much to get the results you need - only 1/4" on each side. you can use a porta-power (which you are unlikely to have) or a ratchet strap hooked to a tree, etc. This is a common way to make panels fit.
