you will need to remove the box to get at the spot welds that run between the box and the cab, remove your tail lights, push the wireing harness down and out of the way, remove the three screws in the filler neck,find all the spot welds and drill them out with a spot weld bit or a 5/16 drill bit, I wasnt worried about damaging my old box sides so it didnt take very long to take them off, dont forget the inner fender support inside the wheel well and the rods that support the corners of the wheel wells, once you get the welds drilled, fight with the boxside, curse and swear on it and then drill out the two spot welds you missed the first time, after that they should lift off pretty easy, take some time and position the new boxsides on the box where they fit best, I used 3/16 large flange rivets to hold the fiberglass on to the inner box about every 8". It took me half a day and 7 or 8 beer to do mine with a friend to help with the lifting in my driveway, your gonna need to fab some supports for the corners of the wheel wells, and leave out the beer, your gonna want to drive it after you get them on. take your time removing the factory sides if they are in good shape you can sell them pretty easy, I cant remember if I had to remove the tailgate hardware from the box or not? you might have to.
Jon