A couple of thoughts:
The threads of the carb upper casting might have some deterioration due to rusting and age. Back off the strainer 1/2 turn and see if there is wobble.
The gasket material is not in itself able to stop the flow of fuel.
I have a set up to test the fuel shut off and have found this area to be prone to leaks. I have had to put some thread sealer on there and/or really snug up the strainer.
Make sure the casting is not cracked from over snugging the strainer
Jerry