There's a 1947 farm license plate on the box I recently hauled home so you can tell how long the farmer used it. He must have hauled a lot of rocks with it though judging by how badly the floor and sides are bent up.
Mike, the taillight bracket is made from 3/16" material. It's 4 3/4" wide; the horizontal flange is 4 3/4" and the vertical flange is 2". The bottom corners are 1" radius and the center of the bulb holes are 2 1/2" apart and 1" from the bottom. The bolt hole centers are 2 1/2" apart. The bolt hole centers in the box are 4 1/2" and 7" from the side of the box and 3 1/4" from the rear of the box.
Do you have the measurements for the step that goes below the endgate?