I've read cork is the only way to go on these trannys. I just put a rubber one on my AOD and it leaks terrible even after beating down the high spots where the bolt holes are. I havent been able to find a cork gasket though.
Go for the cork for sure. In fact, like valve covers, you can silicone it to the pan (overnight) turn it over on something flat and put a cinder block on it. Can even be re-used a few times. Be sure to flatten the edges of the pan on a flat surface first. And, if you followed the above, do not over-tighten and it will work great. Use a 1/4 inch ratchet to insure over-tightening does not occur.
I used the rubber pan gaskets and they are acceptable and can work successfully. However, if a leak does develop, and you go to tighten the pan, they usually deform and leak more. The straighter you get the pan to begin with shall determine if it leaks, etc.