The million topics how GWVR is calculated are not getting the message.
So 1 more time, the factory rates GWVR per tax bracket buyers are willing to pay. In most states the 15,000 lb is the main step in fees and requirements.
So both F350 and F450 are rated at 15k, but since the bigger wheels, bigger axles , bigger brakes on F450 weigh more, when you subtract it from GVWR - you end with less so-called "Payload".
Those are all paper numbers and have very loose connection to actual vehicle capacities.
The only sense making way is to ignore GWVR and check axles rating, including the tires.
Than with big TC the main issue is weight distribution and since trucks up to F450 are design as "grocery getters" with rear axle 4-5 feet from rear bumper, you might end with F550 who is the first in the series with axle placed in the rear and with cargo carrying in mind.
Have fun sorting all those issues. ;)