"Ah. So 6 abreast economy on B747 is not serious, for some reason?"
I tried to specify economy class once on 744 upperdeck yrs ago with a big airline, limiting factor became the luggage stowage. Seatwise you can accomodate about 60-64 passengers. Logistical (galley, crew (de-)boarding but speciallly luggage wise) you create an operational drama.
"If you stick to 9 abreast main deck then you cannot make your plane
bigger than 777-300, basically."
Praticle lenght is limitted to about 80m. As you noted a very wide cabin doesn't seem like a good idea. To go beyond 400 seats with real product specifications an extra deck becomes the next best option IMO.
Boeing looked at it in the nineties..
http://s191.photobucket.com/albums/z..._246Ctotal.jpg
I think the A350XWB / 787 and A380 also do not have circular pressure vessels which are threoretically ideal. However this is not the only factor determining weight & operating costs. If you make the nearly circular damage can be limited.
Like groundloop mentioned MRO companies made good money with Section 41 modification lines.
P.S how do you do Quotes on this forum?