Did this a few years back . The frame rails where done at the other course to protect the box from the guys loading them up with the loader . I use that Idea where I can quite often . The tailgate I cut the side latch rails off and welded angle iron to them and used 2x6 woolminized lumber . Worked out great . No more bent up tailgates . Occasionally you may have to replace they top board when the back hoe takes a bite out of it .