That seat riser definitely provides some stiffness to the floor and helps keep it from sagging in the middle. My suggestion would be to install a channel below the floor where the front of the seat riser was. And possibly something in the location of the rear partition (that's partially gone in your truck). I'd suggest something called a hat channel (~|_|~ ) spot welded along each flange to the floor. Something about 3/4" deep could be installed extending between the frame rails. IIRC, the floor is supposed to be 3/4" above the frame, so you wouldn't want it close enough to hit the frame rails. You could also use that as reinforcement for your seat mounting.
