The tie-break with Miami has yet to be determined. Record against the opposing conference is not the determining factor. It's actually:
"Better winning percentage against teams eligible for playoffs in opposite conference (including teams that finished the regular season tied for a playoff position)."
The Spurs have clinched that tie-break with Indiana regardless of how things finish, but Miami can finish with a better record than Spurs on that basis.
See the tie-break rules at the bottom of this page:
http://www.nba.com/standings/team_re...=iref:nba:gnav