I can't get broken mercury outboard mount off. Please advise

[OrangeCrush]

I have a 2002 15hp Mercury 2-stroke on my Pearson 26 in Brooklyn, NY.
My outboard was broken off its mounting bracket by debris during Hurricane Sandy (Overall I was very lucky, and my heart goes out to all those who lost their boats and homes).

I am trying to get off the broken swivel mount so I can replace it, but I can't figure out how to remove the swivel head that holds it on. The bottom part has bolts and was easily detached from it's rubber side gaskets, but the top part does not have bolts, and the swivel mount can't be slid off because the upper anti-cavitation plate blocks the swivel mount from sliding down off the swivel head.

Since this is hard to explain, I attached a photo. The rubber gaskets that came off easily are labeled "B" on the drawing, while the ones I can't get off are labeled "A"

Sorry for the technical question but this is not covered in my Seloc manual and I don't want to damage something by simply attempting to pry it off. Does anyone have any experience with this?