I would not use a gooseneck adaptor with a fiver. Just look at the frame on a real gooseneck trailer, much stronger than a fiver frame. GNs aren't worried about unit height and the distance between the main frame rails is less. But for a fiver to have decent bedroom height, the frame rails are a full 8 ft apart and the crossmembers are minimal height, just not the same strength. A GN adaptor adds about 14-18 inches additional leverage on the fiver frame, something the frame is probably not designed for.
With my volunteer work, there are times I have to move a 38 ft GN tool trailer. Hooking up is more difficult since you can't see the ball from the driver's seat and you can't be off by more than about 3/8 inch in any direction, so a spotter is needed or lots of trial and error. Plus you need to climb into the bed to remove or install the safety pin and safety chains are required. By comparison, I have been using a standard fifth wheel hitch for 21 yrs, it is a solo job.
Just because your truck came with a GN ball already mounted, if you don't have a need for both GN and fifth wheel towing, why cheap out with the wrong hitch? If you need both, then the B&W companion setup may be the way to go.
Texas Baptist Men-Retiree Builders member since '01
13 Silverado 3500HD D/A, 2wd CCSB srw, custom RKI bed
11 Cruiser CF32MK