John Tvedtnes presents evidence for Adam being Michael, the Ancient of Days, and Heavenly High Priest from ancient sources.
John A. Tvedntes, "Chapter 22: Ancient of Days," Joseph Smith and the Ancient World (unpublished), 213-19
Summary
If any of Joseph Smith’s teachings would have seemed radical to his contemporaries, it was surely his declaration that Adam was the Ancient of Days mentioned in Daniel 7, that he was also the archangel Michael, and that he held priesthood “keys.” None of this makes much sense in the Christian world of today, yet we see that each of these points is supported by ancient texts (to which Joseph had no access) and by modern scholarship.