[link]tt0064782[/link] is indeed an actual movie.
In fact some of the things stated by Homer and Bart, about the movie, are true. It does star [link]nm0000142[/link] and [link]nm0001511[/link]. It was directed by [link]nm0517597[/link]. And it actually is a musical.
However, the song that the characters sing, ("Gonna Paint Your Wagon") is entirely fictional.