Richard Castellano improvised the line and should get all the credit for it. As for the "don't forget the cannoli" line - it depends on who you ask: Coppola says he wanted it in the film and Sheridan says the line was looped in afterwards, because of Castellano's improvisation. She wrote a crazy book - "Divine Intervention and a Dash of Magic" in which she totally goes off on Coppola. Anyway, if you want to hear what she says as to why Castellano isn't in GFII, you could check it out.