I just remeasured an extra plank I had. Width is exactly 7.5", thickness is 1/32" over 3/4". For your bed length It could be fraction of an inch shorter or longer than 78".
Do the measurement like I said. Next to the inner fender wheel well area panel- tape from end of bed panel to back bed brace where the tailgate folds into and the wood would stop. The planks that go next to the inner fender panel have to be cut with a jig saw/ band saw. Put the plank under the bed side protuding out towards the tire from the inner wheel well plank hold down strip that is factory welded to the side panel. Scib around the inner wheel well for the cut you have to make. It's best to put all the planks in the bed to get the right size cut on the side panel planks. Remember to leave about 1/4" to 3/8" gap between each plank for the proper bolt/hold down strip clearance. The router top cut on the planks edges can be increased or decreased for the proper fit of the width of the bed when all the planks are set in the bed. This is where you would take that measurement.
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