A few points about the Montanas.
Plans to lay them down in 1941 had to be shelved due to industrial bottlenecks. In Dec 1941 it was planned to lay them down between Nov 1942 and March 1943 with build times of 3-4 years at peacetime rates of build. But the design was still being tinkered with in mid-1942. By way of comparison the Iowas took 32-41 months to build, partly peace and partly war. Kentucky when laid down in March 1942 was scheduled for 30 months.
So even if you can lay them down mid-1942 it will be into 1945 before they arrive in the fleet.
In early 1942 FDR felt the war would end around the end of 1945. He was reluctant to agree to build ships that would complete after that. That was a factor in delaying their construction in 1942 when there were higher priorities. Any decision to upgun them to 18", a calibre previously rejected, will mean designing a gun from scratch causing further delays (there was an 18"/47 test gun produced by relining and shortening the barrel length of a 16"/50 but it was nowhere near a production standard weapon). And then the ship would redesigned around that new gun leading to more delays.
The USN knew the Japanese were building new battleships and that they would probably significantly exceed the Treaty limits. The surpsise was by just how much and the 18" guns. But despite the USN having intelligence about this by Feb 1943 it did nothing to change the cancellation of the Montanas in July that year.
So for the Yamatos to affect US battleship policy I think that knowledge about them would have had to have been available around 1940.
Here is an article on what the USN knew and when. Tzoli, note the following tantalising line:-
"By the spring of 1943, CINCPAC (Commander In Chief Pacific) Intelligence listed the ships as being armed with nine 17.7-inch (45 cm) guns (the source of this information is not discussed by Prados)."
Circulating that kind of information in an official document suggests a bit more than a guess given everything else that was being said about them at the time. Hard evidence then became available in Feb 1944 but it seems no one wanted to believe it.
[URL unfurl="true"]http://www.navweaps.com/index_tech/tech-084.php[/URL]
