If you are experiencing leaks, I'd say you trans pan is warped. In the past I had a pan on a '65 Mustang C4 that no amount of sealant would fix. I think the sealant wouldn't work because it's impossible to get the mating surface on the trans clean and dry of fluid. After attempting to get the steel pan to seal 3 different times, I finally replaced the pan with a cast aluminum one. Never leaked again.
Cork gaskets may seem like a better solution than the composite/rubber gaskets because they are thicker looking, but they also compress over time and require frequent re-tightening.
I use Ultra Grey sealant on rear diff covers (with no gasket) and have never had a leak if you follow the directions on how to use it.