I believe this subject was the topic of another thread. To reiterate though, I don't think that GF audiences would have tolerated Clemenza as anything except the penultimate loyalist. Clemenza in the traitor role would have been, at least, disturbing, if not an outright insult. The story line could have remained the same with someone else in the traitor role.
I sort of thought this was discussed before. Was there any mention of who would have been the traitor? My best guess would be Willie Cicci.
