samcrimm- Good find on that hitch picture, I've never seen anything quite like that. Looks like a very elegant, and expensive, solution. Probably doesn't sound so bad vs. the total price when that guy is ordering a shiny new Powerhouse coach though.
Take a look through the trucks here:
Home | Heavy Haulers RV Resource Guide
those guys all haul 5th wheel campers with hdt's, and there are a few wild frame stretches on there to put the 5th wheel well behind a big box. Setting up for a gooseneck is essentially the same.
My concern on your project is hitch weight. Yes, your truck is rated to tow 40k trailer, but what hitch weight? Industry standard for a tag type trailer for proper weight distribution and handling is 10% of total trailer weight, but for a 5th wheel or gooseneck trailer that number is 20%. So even if for arguments sake you could hook your gooseneck right where the existing ball is, you may be over weight on the hitch weight. Then if you figure the additional leverage of putting that ball at least 4' farther back behind the truck (and farther from the fulcrum point of the rear axle) the worse that gets.
My suggestion is you need some solid advice from someone with better math/engineering skills on how all those lengths and weights workout before you start modifying anything. You'd really hate to find out the balance is all wrong at 75mph the first time you hit a bad curve or crosswind.