Interesting idea! Though I think that had Japan been annexed, it might've been divided into Hokkaido, Honshu, Shikoku, Kyushu and Ryukyu/Okinawa, seeing as how diverse it is. Honshu might even have been further divided into its regions: Tohoku, Kanto, Chubu, Kansai and Chugoku.
Glad you like it, and you're probably right! Japan's 1950 population was so large it rivaled the most populous US state, New York's population four- to fivefold. Just imagine how that would translate into the House of Representatives! Japan would get many representatives while causing the ratio to bump most other states' down to just 1... 3 at most. Partitioning an annexed Japan into several smaller states would definitely be wise.
Without a doubt though after the war there probably would have been a lot of folks moving to the lower 48 as the standard of living would have been a lot better.
I wish we'd just set a limit of 50000 people or so per House seat so we could get something closer to truer representation for the people instead of capping the number of House seats so it becomes less and less of a representative body as time goes on.