If either Bob Taft or Richard Nixon clinched the GOP nomination that year, I’m almost certain Truman would’ve ran for another term. He was already considering it for a while until literally one of the most popular world figures at the time became the GOP nominee.
Truman believed he could win in another upset against a particularly conservative and/or inexperienced opponent. However, we’d probably start seeing a quicker collapse of Dems in the South since he would probably campaign more on civil rights than either Nixon would or Stevenson or Ike did IRL. We would also see Truman successfully tie Nixon to controversial Sen Joseph McCarthy during the campaign. The names of the VPs are irrelevant in this election as they wouldn’t make much of a difference in the results at all. Let’s say Nixon nominates a more elderly statesman but this doesn’t help him.
Truman wins his last term in a close race getting 267 electoral votes, just above a majority and Nixon gets 227 and a southern racist gets the other 37. The popular vote is even closer, with Truman winning by just about a point. There are a few credible allegations of voter fraud in some states, but Nixon opts out of a recount in those states.
I agree that Harry could have pulled it off, but I disagree with you on his path. I think he could win by basically emulating Carter's map, as there probably would have been a coming home affect in the south (even Stevenson won the deep south) and improving in the Northeast.
