I would not use an apostrophe. I think in this case "seventies" can serve as an attributive noun.
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AnonymousI am confused. Isn't it possessive case? Hit song of the eighties is eighties' hit song?You can analyse it that way, but in practice the apostrophe seems over-fussy. It's a bit like saying "bus's stop" instead of "bus stop", for instance. At least, this is my opinion.