You are asking a bit of a loaded question.  The AOD with a
small block bellhousing pattern is what you need for the
351c.  This is the same patter as the 4.9L 6, the 302/5.0L
and the 351W/5.8L engines.  The AOD should be modified to
accommodated the torque of the 351c and you will need a
linkage to operate the TV cable of the trans.  This
regulates the line pressure of the AOD and can result in
either hard /non shifts or light early shifts.  In the case
of the light shift, it may be a result of low line pressure
with will result in a failed trans shortly.  You will likely
have to fabricate the linkage for you carb.  Baumann Bros
over in Anderson, SC may have a solution for that.  A
properly built/modified AODE or 4R70W with electronic
controls would be better and maybe easier to setup without
the fear of low line pressure because of a bad/poorly
adjusted TV cable.  The 4R70W is a better choice because it
has better ratios to use with your 351c.  Bauman Bros and
others have electronic control setup available for you.

You olds can be updated with a 700 series trans with the
proper modifications.  It also will require some electrical
components to operate.  These are also readily available. I
believe you may be able to get tem for Summitt Racing or
Jegs.

Hope this helps a bit
Lugnut