Montanas would likely survive into the Cold War reserve

What postwar career might the Montana class have had if built instead of the Iowas?

The United States would probably retain its newest battleships in reserve and reactivate them when Cold War crises created a need for heavy gunfire and fleet presence.

The Montanas would probably not disappear immediately after 1945. The United States retained modern battleships because later crises could make them useful again. A completed Montana class would represent the newest and most powerful part of that fleet. Their likely story is therefore long reserve periods punctuated by selective reactivation rather than continuous frontline service.