[luvbowties]

Hello, mweishaa,too bad painter tossed original sills! These bolt-in kits tend toward a jig-saw puzzle when spacing, interchanging boards, strips etc. to adequately cover all the bedspace! I've got a couple of ideas where probs might have come from. 1st, is yours a 3/4 ton which maybe has different hole-design or # bolts going all way thru wood? Maybe it uses a rubber pad(between middle sills and u-shaped supports) with only short bolts up thru support, pad, and then the sill-flat, with a nut on top of the sill-flat? 2nd, there maybe more differences between fleet-and step-side sills than we detected? 3rd, maybe a difference between gmc vs. chevy; i.e., like in some even-same-year-apps, gmc only offered leaf springs where chev had coils.
Idea: since u already drilled holes from the top & down thru the wood, who'd ever know if you epoxied or butyl-rubbered some cutoff bolts into those holes--only the pretty bolt-heads would show?!?
Hope this is not too much "rigging" to suit you. Luck with your project.
sam

*Sorry: somehow I got the idea you bought gmc sills to go on a chevy; still, the ones u got may be from a chevy, like too close to detect a difference.