Search your library for a WAV file named "old traffic". This is a cliche, very often used traffic sound that your uni might have on their drives. It's got a great little "beep beep" in the beginning of the file that I hear at least once a month in dated TV shows and movies.
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To be honest, walla sounds from the 50's don't sound all that different from regular walla's, when listening to the voices. What makes a great walla is not the individual voices, but the sum of it's parts. The thing that makes it sound specifically 50's are mostly contextual sounds, like the car honks that utopia refers to.
Focus on getting good 50s specific street sounds (incl. some voices by street vendors from that era) and everyone will believe it's the real deal/authentic.