Now this is the problem. 58m long and a 4.2m high fuselage would be skinnier than the 757-300.
Going to 2-4-2 and hitting the 228 and 267 two class values would require 43m and 48m lengths with the future 797-8 then being 53m long. With 4.5m fuselage height gives a nice 11.7 side aspect fineness ratio for the 797-8.
The obvious concern with 2-4-2 is the 797-6 would be too stubby at 4.5m height and 43m length gives a fineness ratio of 9.55. That is actually skinnier than an A320 which is has ratio of 9.08!
So even with 8ab the shortest 797-6 will be skinnier than an A320.
RJAMZ - even if your numbers are right (and they might be we are all just guessing) - how do you get to an 4.5M Fuselage Height with 8W and only 4.2 M on 7W?
Are you assuming a different size container in the belly? If not why would Boeing be so inefficient to add that unneeded fuselage height on 8W?
The 7W will probably need to be a little thicker than 4.2M anyways - that is only 2" more than an A320 - even if they stick to an LD3-45 (I assume it will be a new container to take advantage of the extra belly width plus maybe a few inches higher) that seems like it would be hard to hit that number (4.2M) with the crown height they need in the Middle to account for the overhead bins over the middle seats. It will probably need to be at least 4.3M and maybe up to your 4.5M.
In any case - assuming you are right (and I would still suggest you start with a 737-10 at about 195 2 class to get to an 797-6 at most the same length of about 44M then 48M for the -7 and 52M for the -8) on all your assumptions you still aren't allowing for the possibility that Carbon's inherent stiffness advantages could easily allow for longer fuselages.
BTW - if you are right and 8W is the way they go - that means even shorter than my 7W assumptions - you probably wouldn't pick up much in the front as it would still be 2x2x2 in the front Domestic Business - but it would make 1x1x1 sleepers or possibly 1x2x1 easier - and the extra seat width in the back probably means 3-4 rows less - call it 3M less - so an 8W 797 would be something like 41m, 45m and 49m in -6,-7,-8 sizes.
All seems possible. But remember they are shooting for Single Aisle Efficiency in an Dual Aisle - they will need to be as efficient as possible in there fuselage design (as low and and narrow and short as possible) to keep weight down if they are going to compete in this space with a possible rewing/stretch A322.