First thing that occurred upon looking at Worktroll's 'Yokozuna' ship there... Musashi's launch formed a small tidal wave upon hitting the water, flooding a not-insignificant portion of the residential neighborhoods across the bay. This is going to be several times larger.
So.
1) Where do you build it? Yamato and Musashi (and Shinano) needed special slipways constructed for them, because the ones used for the Nagatos and such weren't capable of something their size. So this thing...?
2) How to launch it? As I said above, Musashi caused some major problems with her launch. What do you do in a case like this? I mean pointing it out towards the ocean sounds obvious, but that means your slipway (once you find a place for it) HAS to point a specific direction, and sometimes that's not the best direction for it to point. (Think of places like Norfolk, Suisun Bay, etc.)
3) How to keep it secret? Japan was all about secrecy, both from foreign powers and from their own people (as a measure towards that first goal). This included some almost-comical measures to keep the Yamatos secrets for as long as possible, and they worked- when the war ended and the signing took place on the Missouri, the official American estimates still listed them with 16-inch guns and 35,000-ton displacements (though analysts were pretty sure that was off by a good portion, no one expected it to be over double!). There's no way to keep this secret though... right? The sooner the Americans, British, etc. find out you're building something stupid like this they'll scramble to build something to match it, so you want to stretch that out as long as possible, right?
Beyond the engineering issues with something like it, that all lines up to say that a ship like this isn't even a reasonable feasibility study like the German 'H' was. This is... well, as others said, a waste of trig textbook space.