How completed N3s could enlarge Yamato

How would one or two completed N3 battleships change the later Yamato class?

Later treaty limits could rise to 18-inch guns and about 50,000 tons, pushing a Yamato intended to defeat two treaty battleships toward twelve 21-inch guns and new dock facilities.

The counterfactual assumes that British N3 construction allows further capital-ship development before an arms agreement, perhaps delaying the Washington settlement until about 1926. Japanese 18-inch ships could then help establish a treaty ceiling around 50,000 tons and 18-inch guns. Yamato's stated aim of engaging two treaty battleships would become a much larger design problem. Twelve 21-inch guns are suggested as one possible response. Such a ship would demand new infrastructure, with the panel considering either extending a dry dock into the mountain behind it or reclaiming part of a bay.