This is just an idea, but if you're going to Maria or Anita you might want to think about doing a song from a different musical that those same actors (from the West Side Story movie) played. Wait--that sounded a little confusing.
For Example: If you're going for Maria, you might want to pick a song from a musical that Marnie Nixon (the singing voice for Maria) song for. She sang the songs for Eliza Doolittle in the My Fair Lady movie as well as Anna in "The King and I" among others.
Rita Moreno played Anita in the movie, so you could also look into other parts that she has played (I know she played Tuptim in The King and I, but I can't think of anything else at the moment).
I don't know if this would work or not, but it's always something I thought I would do if I ever auditioned for West Side. Hope it helps
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