Had the U.S. built the Montanas, they likely would have had similar post-war careers to those of the South Dakotas. Because of their speed, the Iowas were more useful at every job except fighting other battleships. Having built the ships in the late 1940s, the USN would have sold them for scrap in the early 1960s.
The Final Salvo:
The Montanas were designed to fight a different World War II than the one that happened. Had they begun to enter service in 1945, they would have joined an armada of twelve modern battleships, against much smaller expected Japanese construction. The howls of battleship aficionados notwithstanding, the U.S. Navy made the right choice when it cancelled the ships in favor of more useful vessels.