Why Bismarck could not simply take 18-inch guns

Could the Bismarck-class hull have carried six or eight 18-inch guns?

Only after a major hull redesign, because a large displacement by itself does not guarantee the breadth, structural load paths, and support points needed for much heavier guns.

Bismarck had a large hull, but it was not necessarily shaped for 18-inch artillery. The ship would need major redesign because turret loads and firing stresses must be carried through suitable structural points. Finding deck space or spare displacement would not solve that engineering problem. A ship can be large without being structurally arranged for a particular battery.