From a historical perspective, the solution would be to land ground forces far away from the city where the coastal guns can't reach, and then advance on the supply route. If this is impossible then paratroopers, if this is impossible, airstrikes.
The thing is, a battleship of the dimenshions you describe would firstly eat up half the navy's annual budget to build, and a 10th of the budget to maintain, next it would indeed be technically possible to build an amphibious craft with such size, but you would have to cut down weaponry, armour and speed drastically to achieve it. This means you would have to drop the 31.5 in guns, and probably also accept a decrease from 20 18 in barrels to maybe 10 or 12. (By the way, when you say guns, do you then mean barrels, or gun mantlets? if mantlets, how many barrels pr. mantlet?) you would have to accept armour that compares to an ordinary battleship, or alternatively accept a speed of only 10 knots (less than a third of, for example, an Iowa class) but i'm guessing, if you intend to use it like an armoured fortress, speed is of little imprortance, and would thus preffer to sacrifice speed for armour.
You should also bump up the amount of reactors to 6 at least, and prefferably 8 or 10, as the track mountings will create MASSIVE drag, and slow the ship to half it's speed without the tracks no matter how you slice it.
The most important factor however, is that the country this ship is supposed to attack has either no airforce, or complete air superiority has already been established by the US air force, as even a few well placed bombs could still sink the entire thing, no matter how well you armour it up, or alternatively, blow off a track and render it immobile, with (i'm assuming) no chance of recovery once on enemy soil.
Also, you should read this, it has every parallel with your narrative, except maybe that yours is believed to have a chance of success by it's own commanders.