Why not just have a step (or two) up from adjoining areas, and just accept it as one of the minor differences in the "FJ" universe as opposed to the -for lack of a better word- "prime" universe?
What is the standard rise of a single step anyway, 6", 8"? Not too far from the 16" you need.
Also, I may be mistaken, but I always thought that it was only the few feet in front of cargo doors in the anti-rooms that had ramps, because of the undercut of the hull in those areas? I'm sure you've already worked this out, so I may be totally off base here, but just a thought.