Shot some pics.
The subframe sits on top of the factory frame rails. I ran a thin sheet of neoprene rubber underneath so it doesn't squeak. It attaches using the existing holes in the frame where the box was solidly mounted.
The subframe creates a single flat surface for the entire length and also facilitates the frame extension.
Back in the rust zone. I have to trim the rear panel to allow the frame to extend past the back of the box. It also needs the clearance to move as the box will articulate.