I used the whole floor of a recent model full size shortbox including the cross pieces. You could just as easily use a long box
Cut the rear sill wide enough to go inside the stake pockets and the floor slightly wider than the wheel openings of the box. The width of the AD box is about exactly the same as the width between wheel tubs .
Cut the spot welds to remove the wheel tubs and save the flange they attach to. You need to reverse that flange so it points down and turn a flange in the box floor....I clamped a angle iron where I wanted the flange and hammered it over. The box sides bolt to this flange
Trim the rear sill until it fits into stake pocket, I also added some rectangular tube to make this joint stronger where it bolts up to stake pocket
this is the best picture I have right now of the finished box, ropes were used to lift it by 4 cargo rings in corners. I am very happy with how it turned out. Strong and able to do truck things. Nothing to varnish. Low cost.
