I would question the original payload figures. The sticker on my F-150 says 7200 GVWR and 1411 lbs. max cargo. That means the truck weights 5789 lbs. I consider the truck sticker figures to be the ones to work to.
When I deduct 500 lbs. for two adults and a large dog, 80 lbs. for the WD hitch and 450 lbs. tongue weight and there is not a lot left.
You do gain some back with the WD hitch but basically my remaining truck payload is in the 300 to 400 lb. range. In theroy a 3/4 ton F-250 should provide about 500 lbs. more.