The election would also be extremely close, but I tend to think that Mitterrand would win. Before Giscard's surge, he had the momentum, managing to get more than 40% in the first round. A Mitterrand victory was expected by many people, though Chaban was also charismatic.
Mitterrand was above 40% in all but a few polls in the first round, and there's nothing special about that. Remember that his candidacy was supported not only by the PS (roughly 19-21%) but also the PCF (20-22%) and the MRG (1-2%). His 43% was not really momentum or anything, but more like winning the quasi-entirety of the left.
In 1965 with the same endorsements he got only 36%. The left was highly popular in the beginning of 70's, they even got more votes than the right during 1973 legislative elections ! Any candidate who starts with 43% of votes on hand has great chances of exceeding 50%.
So? I can't begin to describe how different the political landscape was in 1965 compared to 1974. (As a slight correction, the left didn't win more votes than the right in the 1973 election, though it did come close).
Mitterrand 's 43% was no doubt a good result, but outside of that plus the random Trot and early greenie, he needed to appeal to voters who had voted for the right in the first round. Which, judging by his performance in the runoff, he kind of did but not nearly enough.
