My best friend recently bought a single screw Volvo semi truck to pull his horse trailer. Keep in mind that the trailer is 43 feet long and GVW fully loaded is around 20K. Anyway, he needed a gooseneck hitch custom built to bolt on the frame rails. A combination of some machining and welding, and the pics below show what I came up with. The area where the ball shank passes through is a full 1.5 inches. The skeleton framework on the botom is 2x2x.25 square tube.

