OK, I guess I misunderstood you, then.

As I said, there are many details in both the novel and films that are based on fact.

In addition to the Meyer Lansky-Cuba GF II subplot mentioned by Don Cardi, there is the Rosato Brothers-Galllo Brothers parallel, also from GF II, the Maranzano-Maranzella and Frank Sinartra-Johnny Fontane Academy Awards parallels used in the novel, and the scandal of the missing funds from the Vatican Bank and the possibly suspicious death of the Pope used in GF III.

There are probably others that I'm not thinking of as well.