I dunno, without Stalin and the fear he created it's likely the US may fall even further behind. For example would Molotov-Beria really care about what happened in Asia for example? Would Kim have enough of a relationship with the new Moscow to even consult them anymore? Mao probably wouldn't which would bring an earlier split. The downside is with no Korean war Truman has longer and less resistance to HIS agenda which would end up gutting the US military and very serious consequences to our future missile and rocket program. One of Dewey's percieved 'advantages' that made the race seem to be his, (and allowed it to be somewhat close in any case) was the perception that Truman was being played by Stalin as he seemed to stumble from crisis to crisis. Talking 'plain' and 'tough' during his train tour is arguably what got him re-elected as he was far better getting his point across in person than through the media.Anyways, back on topic.
I have been thinking more about how you actually get the US to Mars by the end of the 20th Century. I think what may be needed is a very different Cold War. Let's say Stalin slips on a banana peel and experiences a fatal fall at some point in early 1945. Whoever replaces him (initially I would bet on a Molotov-Beria double act, but either or both could be sleeping with the fishes by the end of the power struggle) plays the early part of the Cold War much more cautiously and cooperatively (for example, the whole of Eastern Europe, while forced to be Soviet allies, have Finland-level control of their internal affairs). As such, while relations do deteriorate, they do so much more slowly and with much less fear that the Cold War will turn into a hot war - instead the fear is much more that one system will out-compete the other. This means military budgets are lower on both sides, and the desire to demonstrate that their system can lead humanity into the future is much stronger. Meaning there is more money for space exploration, more percieved utility for those in power on both sides, and enough time to plausibly arrange the state of both the Soviets and US rocket programs so that both can be competitive. (Both sides spent enough on nuclear warheads and ballistic missiles to pay for more Mars programs than you could shake a stick at.)
Keep in mind our rocket program was pretty much dead by 1948 and getting worse as Truman ramped back up with the cuts post-48 election. (The Air Force had to make choices with what little money they were getting, and they were bound and determined to see the Army and Navy broken which was fine with Truman) No Korean war and/or a lower grade Cold War and WVB and the Germans are gone by the early 50s. People seem to forget 90% of the "Paperclip" people were only supposed to be in the US for about 5 to 8 years or until their 'knowledge' was considered 'stale' enough for them to be released back to Europe. The US military was in fact not using them at all and they were hardly consulted or given real work to do. (In fact American work had progressed beyond their own work in several areas before the budget cuts came along)
And would Molotov-Beria see rockets and missiles as much as force multipliers as Stalin did? Would they instead fall back on long-range aviation or push for 'allied' forward air bases? A less intense Cold War may in fact greatly degrade interest and/or support for rockets and space travel rather than open opportunties for it