Frame stiffener plates could be added to the top and bottom flanges of the formed c-channel frame rails of the truck, starting a couple feet before the bed/cab juncture, and continuing to just beyond the frame rail kick-up, just before the front spring mount for the rear springs. That would stiffen up the middle of the frame and reduce the flexing in the middle of the truck.
My '97 F350 DRW 2-door cab/chassis, 161" wb, has stiffener plates on it's frame as described above.
My '92 F350 SRW crew cab long bed pickup, 168" wb, doesn't have the frame stiffeners.
The cab/chassis truck's frame is stiffer than the pickup's, carrying similar load weights on the trucks. Both trucks appear to have similar frame channel size and overall frame dimensions, from back of cab, forward. The cab/chassis frame width is narrower, from back of cab, rearward, though the frame channel size appears to be similar, as I remember it.