FYI, my understanding is that Lawrence Olivier was the other strong contender for the Don Corleone role in Godfather I, and supposedly a screen test exists of Olivier as Vito and those who have seen it say he would've given Marlon a serious run for the money.
And I hate to be contrarian, but IMHO the second Godfather movie is the best of the three.
More DeNiro, less Pacino; always a good thing.