This is an interesting question. Before proceeding to my answer I'd like to spend a moment analyzing the options you provided, if you don't mind.
1. How does one "impose" a Pax Americana? I'm guessing you modeled the term "Pax Americana" after the Pax Romana, correct? If I remember correctly, the Pax Romana had more to do with circumstance than proper leadership. I mean, the Pax Romana occurred right after the reigns of Caligula and Nero, the two most psychotic of the Roman Emperors. It couldn't have been their marvelous set-up job that started the Pax Romana so I just don't see a Pax Americana coming to pass in the forties and fifties. Perhaps I would just like to hear more on what your idea of "Pax Americana" is, precisely.
2. Ah, containment. I haven't read my George Kennan in a while but I still remember the broader points: domino theories which gave way to brinkmanship, etc. This, if I recall correctly, also involved the United States propping up unpopular governments in foreign countries to stop the spread of communism. Sorry, but I just can't get behind that ideologically. If the only way we can survive is to destroy that which we are, what is the point of survival?
3. Sounds nice, but getting there would be tough. Marshall, Truman, Stalin, and Molotov really blew the Cold War out of proportion; the whole "Ultimate Conflict of Interests" thing was really just two sides in a newly-technologized world where information was flowing freely that were scared of each other's philosophies. There was no absolute necessity for communism to spread across the whole world and there was no reason for the US to do anything possible to stop the spread of communism...unless one power or the other declared that intent first, and then the other followed suit. Who struck first in the Cold War's political maelstrom? You can argue that all you like but you'll never find a really good answer. Personally I think that if brought off successfully this one would have benefited the world the most, but like I said, getting their would be a headache and a half.
4. In 1945 most Americans, Truman included, knew that the last time we ducked out of world politics the world wound up with the Nazis. Rather than doing so again and letting the phrase "Nazis Happen" supersede "**** Happens," I think we all had enough sense not to just bury our heads in the sand again. There are certainly alluring aspects to this, since letting the rest of the world deal with its own damn problems tends to mean that we'd have more time to deal with our own, but in the wake of the development of atomic technology and two world wars it just wouldn't have been feasible for us to ignore everyone else.
I'd have to go with a combination of 2 and 3. Using military force against unpopular communist governments sounds peachy and co-existing as well as possible with popular communist governments sounds pretty nice as well. Remember, not everyone believed what Josef Stalin believed - that the Soviet Union would inevitably conquer the world. That whole idea was a perversion of one of the ideas of Marx and a lot of the Soviet higher-ups (paradoxical, no?) knew it. I think that so long as we kept Stalinism at bay and encouraged freedom, democracy, and capitalism (say what you want about Marshall's politics but the Marshall Plan in Western Europe was well-timed and well-executed) across the board. Places like Cuba, Vietnam, Nicaragua, Venezuela, Chile, China, and Cambodia still would have become communist, but remember, the strongly anti-US sentiment of most of those places (especially Vietnam) only came AFTER we interfered in their governments. The Korean War still would have happened, as the invasion of South Korea by North Korea (and China) was not a popular revolution. The Vietnam War (or at least our part in it) would not have happened, because South Vietnam loved Ho Chi Minh just as much as North Vietnam did - maybe more, considering what Diem was doing to them. Nicaragua wouldn't have been half as messed up as it was without the American government throwing money and weapons at different guerrilla groups every four years. The only hole in my philosophy (that is immediately apparent to me, anyway) is the Cuban Missile Crisis. Any thoughts on how to deal with that?