There is a little seal around the shift shaft that comes out on the driver's side of the transmission, they are notorious for leaking. It is like a $1 seal and NAPA used to carry a little removal/installation tool to take them out and reinstall them, but since TH 350's aren't being made any more KD and other tool manufacturers stopped making them.
The tool is about the size of a 410 shotgun shell and you can find them occasionally on Ebay. The shift shaft gets grooves in it over time and that can cause them to leak too. A trans shop can pull it out and replace it in about 15 minutes too.
If it is your pan, put a bead of Permatex The Right Stuff around it and it will stop it from leaking. Also, look at where your dipstick and kickdown cable go into the side of the pan, those will leak too.
Don