Apoc,
Did you complete the pan replace? What did you end up using to seal it? Any leaks?
2 weeks ago, I did a trans filter change and fluid flush. An old cork gasket came off in chunks. The new filter/gasket kit from Kragen/O'reily's had a rubber gasket. I installed it, and it dripped.
I think the problem was that the pan bolt holes had a kind of "dimple" to them - so that even with the pan snug under the bolts, the pan metal in between bolts lifted away from the surface of the transmission (and gasket) a bit; at least enough to allow a small leak.
I pulled the pan (had to rebuild the trans), got a master rebuild kit and it included a cork gasket. I had hoped for a rubber one, but did not want to re-use the leaky one as it had some creases from the previous installation.
I put the pan on a block of wood, and using two ball pein hammers (round side of one against the pan bolt hole) I tapped the deformed holes until flat. This was to make sure that the entire surface of the pan contacted the gasket and transmission surface when mated, and not just the pan bolt holes.
This time, I also applied some RTV black to the pan, then put down the gasket, and coated the top of that as well. Not too much. I torqued it all back together in a criss-cross pattern.
Yes, I have read some posts that RTV can get into the transmission and obstruct fluid in the valve body. I have also searched online to see what other transmission builders do, and have read enough examples of people using RTV on the pan, that I decided to do it.
The truck has been back on the road for two days, and nothing drips at all. I also replaced the rear main oil seal and oil pan gasket at the same time (a lot more RTV was used on that!) Again, I now have a clean driveway.
Whatever you decide, good luck and keep us posted on your results!