To add a set of 5th wheel rails to your 2016 GMC Sierra 1500, I recommend the B&W Custom 5th Wheel Installation Kit item # BWRVK2503. This set of rails is industry standard which means it will work with most 5th wheel hitches on the market including the B&W Patriot Hitches item # BWRVK3270 or item # BWRVK3255. The brackets are custom fit and install in pre-existing holes in your Sierra's frame. They fit all three bed lengths of the 2016 Sierra 1500.
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