The hitch must be mounted to the frame of the truck, not to the bed. The bed rails of most 5er hitches are in the bed, but are mounted to the frame of the truck. But most of those "industry standard" hitches would be too high because they are designed to be mounted between the wheel wells, not over the wheel wells of a flat bed.
Your best bet is to install a B&W Turnover Ball gooseneck hitch on the frame of the truck, with a hole cut in the bed to allow access to the gooseneck hitch. Include a hinged cover for that hole for use when the hitch is not being used for towing.
Gooseneck Hitch | B & W Trailer Hitches
Then install a B&W Companion 5er hitch in the Turnover Ball.
Companion 5th Fifth Wheel Hitch | B & W Trailer Hitches
The hole in the bed may will have to be big enough to allow the Companion hitch to be plugged into the Turnover Ball. And you may have to cut some of the bracing under the bed. But any good welding shop can fix it up easy.