Hi Knirk,
I replaced the same gasket on my car about a year ago and had the same problem. Unfortunately I can’t remember all the details but I did get the plate off with much fiddling and jiggling, not too much force though. I also seem to remember having to disconnect the solenoid arm at one of the ball joints. Mine has been leak free ever since.
Sorry I can’t remember any more details but hope that helps a little.
Good luck
David Brough
1969 280SL Auto with A/C