Frame flex (or lack of) is part of the suspension. C-channel flexes differently than a boxed frame. Ford's fully boxed pickup truck frames are a lot stiffer than the old ones, but boxing the frame aft of the cab on chassis cabs might have lead to a lot of broken frames. If a boxed frame is pushed too hard, it might break where c-channel might survive.I it probably isn't boxed behind cab because it's easier to fasten to a channel than it is a tube.
This may matter to you for your choice of a frame.
If Ford made a 2017+ F-550 pickup (with a fully boxed frame like the 2017+ 250/350/450 pickups) with a 19,500 pound GVWR and a wide track front axle, would this thread even exist?