Eighteen-inch guns push capital ships toward battlecruiser logic

How would an old six-gun, 18-inch battleship fare against a modern Iowa-class battleship with equal fire control?

The Iowa should benefit from being decades newer, while an 18-inch-gun design would struggle to carry armor equal to its own firepower and would likely emphasize speed and the first hit.

Equal fire control is a large assumption when comparing a 1940s Iowa with a much older hypothetical ship. Protection against 18-inch fire becomes so heavy that capital ships begin to resemble battlecruisers. They would rely on speed, firepower, and an early hit rather than full protection against an equivalent gun.