Puzo gave a detailed physical discription of each brother in the novel, so it would be impossible to imagine even with make-up and dyed hair, Ryan O'Neal or Robert Redford in the role of Michael.
I recall soon after it was announced that Al Pacino had been cast as Michael there was an article in my local paper's Art & Entertainment section with his photograph. I couldn't believe how perfect looking he was for the part going by the discription of Michael in the book. I was glad that Al Pacino is an Italian with Italian features playing an Italian.
I read somewhere that Ernest Borgnine tested for the role of Don Vito Corleone.
I was going to start a new thread entitled "If you were casting Godfather I today" who would you choose, but I couldn't think of anyone. Oh, well its almost 2:00am.