However I think there is possibly another reason why Boeing have not put any more top deck behind the wing, and that is most likely for Weight and Balance reasons. Most aero-modellers know it is acceptable to put lead weight in the nose of a plane or glider, but not to put any in the rear.
Quite the opposite phiggs, many modern aircraft have fuel tanks in the tail for the main purpose of obtaining a more fuel efficient aft CG. There are many reasons Boeing did not build a full upper deck 747, W&B was not one of them.