If on the drivers side it may be the shift lever seal in the case. If on the other side, it could be cooler line fittings, the dipstick tube o-ring, servo cover, and on some later trannies there was a pressure switch for a spark retard. What pan gasket did you use? If cork, it should have been installed with either no sealant, or a light bead of 3M yellow weatherstrip sealer to hold it to the pan. Torque the pan bolts to about 90 inch/pounds. I like the Farpak or Accopac gaskets myself, and install them dry as well, or with a thin coat of assembly gel lube on each side. Cast pans have a few benefits. They can add fluid capacity if you use a deep one, plus they help to strengthen the case since they help avoid flexing like a stamped pan will allow easier. If you need more help contact me!