When I was decided to put a sleeper on my LTL I had to remove the frame mounted stack and move the stacks against the cab to permit room for the sleeper. I obtained a set of brackets from a friend and by looking at pictures of other LTL's with these brackets I guesstimated the approximate locations of the brackets. I took some masking take and taped the areas where the brackets were to be mounted and marked them off with a magic marker. I center punched the marked holes and started drilling, after drilling the first hole I noticed that it was unusually thick for what I thought was just sheet metal. More like 3/8 to a 1/2 inch thick. Before I went any further I removed the in cab upholstery panel opposite of where I drilled the hole and then took a hole saw and sawed a hole the size of a half dollar so I could see in between the two pieces of sheet metal. When I looked in the hole I could see there was a nut plate that was welded on the inside of where I had drilled the hole. I had drilled right through the dead center of the nut. I had no idea that the factory had installed the nuts for the stacks even though the truck did not come from the factory with the cab mounted stacks. I drilled slightly smaller holes in the rest of the marked areas and cleaned it up with a tap and used the original factory mounted nut plates for the stacks. I would say that my guesstimating was pretty accurate.
With age comes wisdom. With age also comes lots of pains never before felt.