I see from your post that the trailer axles have already been flipped and are mounted below the springs, so there is no other adjustment in that venue.
I don't understand your bed rail measurements, unless there is a typo. You have stated your front bed rail height is 32" lower than your rear, when I think you meant to say 2".
You may want to scrap the over sized tires as they make the truck awful high for fiver towing and reduce your torque.
Lastly, I highly recommend the B & W 5th wheel hitches.
M & M :C On the road again!
2007 GMC 3500-SRW-Duramax-longbed-4X4
2008 Keystone EVEREST 348R 5th wheel
2002 SUNDOWNER gooseneck horse trailer