If a United States bent on world conquest went after the Soviet Union back then, it would have been one helluva fight, atomic bombs notwithstanding, but I can see using that sort of conflict as the best start to try world conquest. Keep Chaing Kaishek in power in China through direct intervention and manipulate "military assistence" into "occupation" and then outright control over time. That eliminates the two biggest opponents. The war against the Soviet Union would work towards setting the stage for a hostile takeover of Europe and would catch the UK and France in a very weak state. So, the hypothetical situation could work. Success in China and the USSR would be the most important points in determining success.
All in all, its a pretty far-fetched scenario. The United States is a historical oddity. A nation that is the most powerful in the world (one of the two most powerful in Cold War era) that does the most illogical thing imaginable. It does not seek to guard the technologies and systems that have made it so successful. No, they actively push them on the world. Kindof silly if you think about it.