I use Cork for a C-6 trans. The best advise with cork is make sure both surfaces are clean and dry. Torque it down once, then go around again. I sometimes go around 3 times. You want to get full crush on the gasket. If there is oil or gasket sealer on the surface where the gasket contacts, a cork gasket will slide and split.
Also, check to see if the pan bolts have dimpled the pan rail. I fix those by putting the pan rail on the edge of the bench (must have the inside of the pan facing up), set the ball end of a ball peen hammer in the hole and use another hammer to hit the flat end of the ball peen.
Mark
Berkshire Transmissions
North Adams, Massachusetts