Quick story, we towed a 12,000lb (boat and trailer weight) with around 1200lbs tongue weight on a bumper hitch. Originally it had 1" shank on ball, but was later upgraded to 1 1/4" shank.
Rest of story, Pa was a mechanical engineer and could design and weld things very well. He fabricated reinforcements under the bumper and attaching to the last 3' of the frame on each side. He used 1/2" Bethlehem (sp) steel plate and grade 8 1/2" bolts with double nuts. It was way overbuilt and handled the load fine.
Generally speaking, factory bumper hitches are not rated to pull much.