Battleship guns cannot be improvised

If the United States learned of the Yamatos while they were being built, would it fit 18-inch guns to the Iowas?

Only if a suitable gun programme already existed; more likely the United States would accelerate proven 16-inch production, with any 18-inch option falling later on a new design.

Yamato, the Iowas, the proposed Montanas, and HMS Furious reveal the industrial lead time behind battleship guns. Large naval weapons require design work, test pieces, manufacturing capacity, and long production schedules. A navy can accelerate work already under way or redirect a programme into a later class, but it cannot simply order a new 18-inch weapon into existence once intelligence arrives. The 18-inch gun on Furious likewise implies prior development rather than an improvised response to one hull. Work begun for the cancelled Lion-class 16-inch guns did not instantly become finished weapons, although usable material could be redirected into smaller-calibre production.