With the earliest PoDs after WWII, but preferably the latest possible, improve the economic power of both the US and Japan without damaging each other (i.e. Japan still takes over the automobile industry, and thrives in information technology). It can be through anyway, whether trade deals, movies, corporations, or anything else. If they can together make up 40 or 50 percent of the world GDP by the present day, that would be great.