There's no need for the platform to be a battleship.
If it's just a single gun and ammunition system, could you fit this onto a small, low-cost commercial hull?
Such a ship would be operating under land-based SAM and air cover for most of the time anyway, given a 1200km range.
Again, how can a 400mm gun with a 1200km range with affordable ammunition have very narrow use cases?
It can range over all sorts of targets over the 1st Island Chain. Seaports, runways, factories, power plants, military bases, etc etc
There's literally hundreds of thousands of potential targets.
There's not actually that many targets in the Second Island Chain.
And until China has a much larger Navy, it's not realistic for such a Chinese artillery ship to get past the Second Island Chain.
That just leaves a hundreds of thousands of potential targets within the 1st Island Chain.
I'm not talking about the supergun itself, but the hypothetical naval platform where said supergun could be based on.
The primary (if not the only) purpose of the monitor is to shell enemy coastal installations and provide fire surpport for allied forces conducting amphibious assault operations against enemy beachheads. Other than that, monitors aren't useful at all: They are very slow; They don't have comparable radar, sensor and computing systems; They don't have sufficient endurance for the high-seas; And they have very weak defensive capabilities. This means that monitors are never suited for high-sea operations, hence limiting its flexibility and viability (and why we don't see a lot of them after WW1).
As for the question of commercial hulls - Sorry, but no.
Warships are specifically designed and built such that the hull structures can withstand the continous recoil loads from the main gun's operations, whereby the forces generated by the gun during firing can be distributed and dissipated effectively across the rest of the ship's hull structures.
On the other hand, commercial ships have no such capability - That's why we don't see 76mm or even 100mm gun turrets on Zhong Da 76. Trying to put the 400mm supergun system onto a commercial ship is no different that trying to destroy the ship itself and making the supergun single-use only.
In the meantime, for China's case of the supergun (and taking the claimed range of 1200-1300 kilometers) - If the supergun is only meant to be used against the 1IC, then China doesn't really need to base the supergun on a naval platform at all. This is because most of its intended targets along the 1IC are already readily reachable with land-based system alone. That's why it makes no sense to base a supergun on a monitor.
If there is a need for a ship-based variant, then the PLAN might as well go further and base the supergun(s) on a proper sea-going warship, where the supergun(s) can be moved across the seas and strike targets that are much farther away from the Chinese coastline.
Moreover (albeit hypothetically) - Who says that the supergun must only attack stationary targets? Who says that the supergun cannot fire guided projectiles against (larger-sized) mobile sea targets, if such need arises?
