I truly believe the box on my 2004 Super Duty is being held up by the drop in bed liner. I have a 2015 box waiting for paint and install but now I will get to repair the bottoms of the doors with it. Still looks better then a friends 2004 Chevy that has less miles and body left on it.
I had seen when I was a temp at Ford Motor Company Buffalo Stamping Plant that a couple of body plugs in the rear wheel houses of the F series truck is all it takes to slow the rust in the wheel arches.
Do this-
Then you can rinse out with hose or pressure washer and fluid film inside. A couple hole plugs and no one will notice.