A true fish plate is not supposed to have any horizontal or vertical sides and all "corners" should have a large radius.(a fish shape) If the splice is done on a angled place on the frame, then try not to have any sides that are parallel or perpendicular to frame angle. All this to prevent stress risers which induce cracks, Even though you don't plan on loading it, it is heavy. Also the recommendation for frame cutting/joining is a "Z" cut. It is not recommended to use thicker material than the frame and continuous welding all the way around is also not recommended. Boxing is for Hot Rods that have different suspensions, different loads. Not necessary IMO on our trucks. Possibly taking away a designed-in allowable twisting torque which will stress the frame. The newer trucks are solid rect. tubes, bent and seam welded/hydroformed (like a box). The modern truck is a totally different animal. "C" channel is still used also.
I can't imagine what must have happened to that truck, is it just one side? Two places? Do you think that they spliced in a piece from another truck?
