In the scene where Albert S. Ruddy visits the Zoetrope editing suite to convince Francis Ford Coppola to co-write and direct his film, a man, uncredited, walks through behind Coppola; he is later seen sitting on a couch in the background with a young woman (and another man, seen from the back) as Ruddy and Coppola continue to talk. This "walk through" man is clearly George Lucas, the woman is likely Marcia Lucas or Gloria Katz, and the other man is probably Willard Huyck, all of whom were friends and acolytes of Coppola circa 1970.
When Barry Lapidus (Colin Hanks) says Paramount's last gangster film was a "sugarless turd" he is referring to The Brotherhood (1968). Because of its poor box-office performance, Paramount had stopped making gangster movies, and almost passed on the opportunity to make "The Godfather".
While Robert Evans is talking to his secretary about casting Walter Matthau in the film version of The Odd Couple (1968) (a role that Matthau originated on Broadway), a framed photo of Matthau is visible on the wall behind him.
In the opening festival scene, one of the mobsters takes a cannoli from a vendor's table and is told by another to "leave the (expletive) cannoli." This is a clear nod to the famous scene in the original film where, after the hit on Paulie, Clemenza tells Rocco to "leave the gun, take the cannoli."