A bit before my time, but for more inspiration - didn't the first Rocky movie come out in 1976? I think Saturday Night Fever came out in 1977.
Here's "Stayin' Alive" on YouTube:
http://www.youtube.com/watch?v=u1qN6gLbUMwMust watch this video - it's hilarious! You *will* get some fun ideas by watching, I'm sure of it. In some of the dance scenes you can see some of the styles of dresses women were wearing (no, not prom dresses, but you'll get a good sense of the styles).
Still smiling - can't help but giggle at John Travolta blow drying his hair and then putting on *peach* polyester pants...
More inspiration..."If I Can't Have You" (more prom-like song, imo, and the movie scene gets pretty romantic at the end)
http://www.youtube.com/watch?v=hPaUrBvFZp4Looks like in one of the dance scenes, there's a jumpsuit of sorts - white halter/palazzo pant type outfit. Later on, you see the ruffles etc. on some of the other dresses.