Afternoon,
I would need additional detail on the configuration of 350 dually-cab( regular, king, crew?), powerplant (diesel or gas?)drive (2 or 4wd)? All have a significant bearing on payload #, the door jamb sticker will specifically denote payload of your exact truck as built (options and above noted items). The payload spread according to the Ford 2019 fleet towing guide, I think ranged from about 7,000+ pounds down to sub 5,000 pounds. I don't know the weight of the AF990 but my prior AF 996 wasn't exactly svelte!
Timbrens, stabilloads, bigger sway bar than stock, shocks, and tires will (in my opinion ) mitigate the felt effects of a particular load but do not alter the actual payload number.
I have taken my Host Mammoth on some dirt tracks off Forest Service roads that some may second think in a full size pick up; however I wasn't winning any speed trials and I fully understand my wear items will be a quicker replace than some other owners may experience.
I would likely do what you suggest except for the speed component, I don't know what you consider highway speeds. On interstate pacing fastest out there or running the speed limit on a flat straight county highway in eastern Oregon? I tend toward conservative on speed when loaded.