Going by what Ford is saying and the comparison charts. Nothing is calling out the frame as being an upgrade. Just suspension. Talking to chassis experts who upgrade these things for a living helps.
They're all saying you can do most of what a 250 can do with some upgrades to the suspension itself.
I even found a Ford brochure listing each truck and the benefits compared with the other series. Nowhere can I find it said that the 250+ have a beefier frame. They'd sell the crap out of that point if they could. All they talk about is the motors and suspension. And of course dual vs single rear.
Again, no one is saying you can start running hotshots with an F-150 and air bags, but it's not the drivetrain or frame rails making the difference. Most of it has to do with stability and squat. Not power. Not bolts snapping. Not frames bending.