The axle and wheel bearings will handle at least 10,900 lbs. and the tires should handle at least 11,000 lbs. and with the truck at the rear axle putting down a weight of roughly 3600 lbs. that leaves a payload of 7400 lbs. IF the frame and leaf springs are up to the task.
The leaf springs can be upgraded but not the frame. Ford with its chassis cab version of the F-450 shows an additional 3000 lbs. of payload over the F-450 pickup version and there are notable differences in both the frame and the front suspensions of the two models.