Fred
  I mounted the rail-mount right over the "T"....  where the vertical rail support is.
Here's what you need to know:
  The Scotty rail-mount comes with a piece of 1x6x6 plastic board for support.  This keeps the rail-mount from twisting downward on the rail from the weight of the cannon ball rigging.  This works fine, the problem was that when I swung the boom back up into the boat it would pivot around on the rail to the inside and fall all the way to the transom.
  To solve this problem I bought a piece of 1x6x6 oak and put it on the other side so that now I have supports on both sides.  The base of the downrigger is 6x6.  Now I have a "V" shaped support running from the base of the downrigger at the rail, down to the gunnel.  It was an easy fix, only took a few minutes.  Does this make sense?