If you remember, the 2008-2010 F450 pickups had a GVWR of 14,500 lbs, which put them in the class 4 range. Some Ford marketing group probably made a credible case that the F450 pickup was “stealing sales” from the cab and chassis F450’s. The two platforms are intended for a different customer set, so keeping the F450 pickup a class 3 probably helps separate them.
I’ve owned two of them, a 2010 and the 2016 I have now. I bought them because I wanted what you could get on them that wasn’t available on the F350’s. Dana S110 rear axle, Bigger brakes, 19.5 tires, and the improved turning radius of the wide-track front axle. This all makes the F450 weigh more than the F350, and it’s all unsprung weight.
If you were to buy an F350 because of its higher payload, then put 19.5’s on it, your cost would be about the same as an F450 pickup. IMHO, ignore the GVWR on the F450 pickup. Use the weight ratings on the tires at max psi. Don’t overload the tires and you won’t overload the truck.
I’m not sure about the 2017 and up F450 pickups, but the previous generation trucks like mine may squat more than you will like if you put more than about 5500 lbs on it. That’s a common complaint from those who haul or tow heavy with them. There are several options available to resolve that issue. I’ve got upper Stableloads on mine. It’s really the F450 pickup’s only shortcoming.
:):)