I'm definitely calling Moss as soon as I get a chance. I was just talking with a friend that used to have a 65 MGB and had experienced the issues. He recommended using Permetex on the seal to keep it from deforming later. I'm not convinced, cleaning up Permetex can be a chore especially in locations like these. I think that finding a wider seal from say a Fram filter mentioned above may be the better solution.
As for a locking tab, is MGAdavid referring to a lock washer, or star washer? As that wasn't part of the kit or in the directions, I'm not likely to volunteer to be the tester on that. Steel lock washers or star washers would chew up the aluminum adapter plate. Again, a call to Moss is in order to see what they recommend.