To my knowledge, none of the rubber manufactures offers a 63 to 66 A-Body convertible windshield gasket. That being Precision, Steele or Clesters.
I would imagine finding a NOS one will be equally difficult to locate. You might check with AMS Obsolete or Len Dawson. If you did get so lucky to locate a NOS piece, you might want to talk to Steele Rubber to see if they might want to look at it for the sake of reproducing the gasket.
Good luck with the search.
