the whole Godfather "thing" is based on real facts and Mario Puzo's stories are connected with each other (I heard that Michael Corleone appears in another MP's book). many things in the trilogy are based on real facts, like the the organization of the Corleone family and the other families, Fanucci and the Black Hand, the shot on Moe Greene (I know some real mobster was shot in a barbershop, but I don't remember who) and many other things that many of you know better than me (I just started reading about real life mafia).
so I believe the storylines from GF2 and 3 are original but with a lot of quotations to other stuff.

PS: anyone see some connection between John Gotti and Joey Zasa? them both took over criminal empires (Gambino and Corleone family), didn't last too much with the power and were celebrities.

Sorry if my post was a little (or totally) off topic