Do you know the manufacture of the hitch if so, contact them. Tork lift in Kent Washington has/had a few setups as you describe. They included a deduct for extended ball mounts.
If your near here, easy to get. If not, email, ask if they can do a hitch as scooby described. Easy to put on a pallet, ship to you ready to bolt onto your truck. Or in multiple pieces, assemble them one at a time.
On edit
Their are probably dozen places or more in Puget sound where I am that can custom build hitches to do as the customer wishes. Looks like you are in the Midwest. I would wager a small sum that you are at max, a days drive from an equal body bed hitch manufacturer. They can and will build a hitch to your specs. Not just a generic number.
Marty
