This was the first I'd ever heard of James Dean, when I saw it in the 70's as a teenager. The fifties were not popular with kids at the time. It came out pre "Happy Days". That is why, I believe, the 70's music was used. It's an attempt to draw kids into the James Dean Myth, and the fifties in general. At the time, I loved the music and still remember a part were the song "James Dean" by The Eagles was playing as Dean walks around a race track. It's my first memory of Dean. There wasn't much about Dean available at the time. No VHS, DVD, books or youtube. This made him seem much more mysterious and iconic then he seems to me now. Wish I could see it again, though I'm sure the music would seem dated and it wouldn't have the same impact, with all the Dean material available today.
