I finished the custom box today on my 54F100 project ( Miss Fortune ) here some pictures, sorry about my travel trailer being in the way.
blending the rear valance into the ends of the bumpers was a real challenge. I used 11ga 3 X 3 angle and had to add 1-1/4 inches to the bottom to fill the inside of the bumper. formong the curves took the heat wrench and a lot of hammering hot metal, while welding the seam together.