To add a bit to what bullfoot said, do not put anything you value between the bed frame and the truck frame. To fill the cylinder, crawl under the truck frame and raise up between the rails. Use a 4x4 or something solid to support the bed, and be sure the bottom end won't slip. We used to use a forklift, but I doubt if you have one handy. This is a job done best with two people for safety's sake.

Fred
52 3600
69 C-10