Some reasons why I've come to think that the Bocchicchios are the probably (not to a mathematical certainty) the fifth of the Five Families:

1. The other New York families seem to have territories in Upper New York State, New York City, and New Jersey. The Bocchicchios seem to fit pretty nicely in there, with a territory in the Hudson Valley.

2. They're given a detailed description in the peace conference chapter, just as the operations of Cuneo, Stracci,Tataglia and Barzini are described. It would have been silly to hold the description of the Bocchicchio operation until the New York dons actually arrived, because the long narrative description of the Bocchicchios at that point would have sidetracked the description of the peace conference.

3. The Bocchicchios are largely played for comic relief, but they are a substantial family. They're not just vendetta artists: they negotiated the terms of the conference. And they're powerful enough that they can project fear as far as the West Coast. Also, from Vito's statements, it seems he respects the Bocchicchios more than, say, the Tataglias.

4. Finally, I think the biggest stumbling block is just the mental dissonance between one family being both part of the war and also negotiator/hostages. But Vito had to make the first step in some direction. Who else could he turn to -- the police? It has to be either an outside family, or one of the Five Families that he trusts.

From the outcome of the novel, it's obvious that the Corleones only have a blood feud with Barzini and Tataglia. There's no reason to think that they wouldn't turn to any of the other families to set up a truce. And from all that is said about the Bocchicchios, it is clear that, if they agreed to a truce, they could be trusted to abide by it. They don't lie.

I don't have the book in front of me, and I have a couple of hanging questions: 1. does the book say that the Bocchicchio negotiator arrived at the conference early; and, 2. does the narrative of the Bocchicchios support their being one of the "six powerful families" by 1937?

These are just some things I think about when I should be working.