Oli, I've never seen a direct quote from Puzo on Johnny Fontaine/Frank Sinatra. But, it's a good guess that since he borrowed and adapted real-life Mafia characters and events to the novel, Puzo was repeating old stories about how feared NJ caporegime Willie Moretti coerced studio chief Harry Cohn to give Sinatra the part in "From Here to Eternity" that resurrected his career. That story isn't true: Cohn gave the part to Sinatra because his original choice, Eli Wallach, "wasn't skinny enough." It was also rumored that Sinatra sang at Moretti's daughter's wedding.

As for Jules/Lucy (one of the most boring and unnecessary parts of the novel, IMO): I've long believed that he included it so he could describe her gynecological operation in detail. I'm betting that one of Puzo's female relatives/friends had the operation. It fascinated him, and he couldn't resist putting it in the novel, in detail--just as he included all that Hollywood BS in detail.