I can't imagine it any other way than it is. Caan is Sonny and DeNiro is young Vito. It's as simple as that.
Thats how i always see it as too, I think Caan was made to play sonny and De niro to play young Vito. I personally agree that De niro was too crazy in the screen test and it wouldnt have looked right if De niro played sonny.
