Me personally? I hear them talking about the flood of Noah's day (when Jesus said, "Just as in the days of Noah") and the 120 years of preaching that Noah did....
I think that if all this insanity of "overlapping" doesn't work out (or, even to support/ reinforce it) they will say something like: "Jesus 'evidently' meant to give us a clue when he mentioned Noah and the preaching' and that will mean this 'last generation' could last until .... 1914 + 120 = 2034! So keep preaching, like Noah did!