I agree that there's justification for the historical backdrop of NYC's five Mafia families, but as I've said many times in the past....

I think that Puzo deliberately made the Corleones a fictional sixth family so as to avoid speculation as to which of the actual five families the Corleones were meant to represent.

That's the point: As a fictional sixth family, they are not meant to represent any of the actual NYC families.

And, strangely, from the very first time I read the book - way before the movie came out, that's the way I figured it.

Five NYC families, and here's a story about a fictional sixth family and their interaction with the "real-life" five.