I just installed a fixed CJ back seat into the factory location and I was a bit surprised by how much space I had behind it. I had wondered if I'd still have room for my spare tire (33x12.5-15). I'm measuring about 37"'s from the back of the seat to the tailgate. Just an fyi.
Prior to paint I relocated the mounting holes for the rear seat backwards a good foot and a half, so that it would press my Jeep box against the tailgate and keep it from sliding around. It's a better fit there than behind the bulkhead.