I like the removal of the stairs.
I always hated the location, the loss of floor space, and the lack of space for the passenger seat.
I would get some frame rails that fit into the original frame rails, and overlap the back 3 feet into the originals. This will give you a solid platform to start building from without having to worry about it being strong enough.
My 92 Ford B700 had 10 1/4 inch tall frame rails.
My 92 TC2000 had 9 7/8 inch tall frame rails.
My 76 Dodge D600 had 9 1/2 tall frame rails.
For the record, no bumper mounted trailer hitch is made to take more than around 600 pounds of weight. Trailer weight is completely different.
"Don't argue with stupid people. They will just drag you down to their level, and beat you up with experience."
Patently waiting for the apocalypses to level the playing field in this physiological game of life commonly known as Civilization