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Using a plasma cutter and then a grinder Mike got the edge straight enough to fit the plates. The boxing plate should be slightly shorter in height than the framerail surface so that you are filling in a small "L" shaped weld surface at both top and bottom. If the boxing plate is the same height as the frame rail, you are not getting the proper weld fill, and you will grind most of the strength away when you do your finish grinding. If the top or bottom rails are not 90 degrees to the sides of the rails, tap them up or down as necessary, or grind a little off of the boxing plates to allow this proper fit for welding.