But to say it isn't an American classic is like saying Norman Rockwell isn't an American classic. I don't really much like Rockwell's stuff. Some of it's ok, but I think it's schmaltzy and redundant. But I'll tip my hat to him as an American classic even though I don't personally like it. He deserves his place in American art and culture, no argument here, as does The Godfather.
I also understand what you're saying about the book vs. movie, but I believe The Godfather movie was one of the very few movies to convey the essence of the book it was based off. We disagree here again, but that's fine.
Also, since they're different mediums of artistic expression (print vs visual and audio), it becomes apples to oranges -- each has to be judged on its own merits.