"We have the 2020 Expy Plat 4x4 w/ Tow Pkg and are kicking ourselves for not getting the extended wheelbase MAX. 5 more gallons of fuel and a foot longer wheelbase....which might make a difference with 32ft 6400lb trailer."
Not from personal experience mind you, but from what I know--that longer wheelbase will definitely make for a better tow vehicle when the weight gets up there. And one of the things with manufacturers tow ratings--there is no standard of measurement (same reason some tires, despite having the same number on sidewall, are taller or wider etc.)
I really believe there is a niche for an F250 styled frame, full 6.7 diesel Expy (albeit a small one), most especially if they TREMOR'ed it. YMMV...