Yep---in this contest, a push is a win for the Under so 53 is an under even if the line moves there...
since I stamp the O/U at kickoff---and people make picks as the line changes---I treat the pick and the SB tiebreaker score independently..
in your case, you've got the under no matter where the line ends up--and no matter what your tiebreaker score is---unless you change before the kickoff...