@FearlessDiva,
Check etymyonline and make sure of the dates when these words were first seen in print. That should give you an idea of usage. Then you can probably figure a few years before would be the true coinage dates. This is because it can be a while before slang reaches print.
If it's a few years (say, the term is from 1980 and your story is set in 1978, you're probably fine. But a term only dating back to 1998 is going to be too far removed from 1971.
IIRC butthead was used in at least the 1985 Back to the Future film. If you're setting the story in 1978, then it's probably fine.
But check etymyonline first.