well, wrong.
the specification of the 155mm said that
it is fitted in the same space as the 8 in. the carrousel would look nothing alike though (this image is directly below yours
the 3 decks depth is most likely including the (hoist to) the magazine. magazines are not included in the belowdeck parts as they vary between different ships. taking wikipedia as reliable source for stuff like this, is also not an good idea, I don't have to tell you that, right?
the same goes for the barrel length, the 155mm is an all new gun.
I can also proof that to you: the 8 in has an caliber of 55, the 155mm of 52. 114*55= 6270mm, 155*52= 8060mm. explains the problem, right?