I personally think that the KC-135 should be a separate product because there are many things different about the two aircraft. My point: The KC-135 has an improved  cockpit, the back of the plane is mot definitely different, and there is no boom control room in a regular 707. This is only the interior. The KC-135 also has different wings, engines, and it's been made larger. Honestly, if this plane were to be an expansion pack, it would be a complete overhaul.