sandra I can see the point of keeping the bed low by building it without a frame do you mean to run the angle leg down along the frame to act as a stiffener.  And I have done one like that for a friend then added a B&W fifth wheel hitch worked out real slick but I would consider the angle a frame.

So the angle iron is the frame right, same thing I did on mine angle iron capping the frame of the truck, with cross members welded flush with the top of the angle iron, these beds are not frame less there what us fabricators call a true low profile bed.

A frameless flatbed? that would be like a boneless human?