There's still a little bit of room in the southeastern corner of the airfield where Boeing has its current facility to build an assembly building or two. Dreamlifters use this area to do their onloads now as it is.

Also, the Japanese may have extremely efficient plants, but their labor unions are extremely powerful and influential, even more so than their counterparts in the US. I used to oversee an aircraft plant near Tokyo that did work on US military aircraft. If we wanted to accelerate schedules or have the company support flight ops on the weekends, they had to clear it through the union first.