Assuming we are talking aluminum torqueflite transmission, at least, leaks are not hard to find (I am not so sure with iron torqueflites).
If it is the front pump housing seal, you will see fluid drip at the front of the transmission case and it will be wet behind the lower metal cover. Usually if the pan gasket is fairly recently done, then the most likely culprit is the selector shaft seal. To determine if it is the latter, clean the area on the ledge above the front driver side corner of the case until all the dirt and and oil residue is gone and that small area is clean and dry. Then let the car sit for a few hours to overnight depending on the size of the leak and then feel up in that area to see if there is any red fluid wetness. If there is, then it is most likely the selector shaft seal. In my experience,the selector shaft seal is most often the culprit with the trans pan being a less likely source of the leak. If that area is dry on the ledge that you cleaned, then it is likely the pan gasket still, but if that small area is wet, then it is the selector shaft seal. It can be changed in the car in about 20 minutes once you have the proper tool and technique in using it to R & R it. I have never had the very small seal leak at the top of the selector shaft housing, only the larger selector shaft seal at the base of the housing at the transmission case ledge, so that is the only one I would replace until you do a full rebuild. If you try to replace the small seal at the top of the valve body selector shaft, it is often a little difficult to remove the e ring holding it together, and then if you do remove the e ring, you risk having the kick down shaft fall into the pan and then you generally have to pull the pan to get things in place properly for the transmission to operate correctly again. So just replace the lower, larger seal if doing the transmission in the car.
There is another potential leak source at the rear of the case due to a failing O ring on rod that one of the band levers pivots on. But it is less likely to fail. It too could be replaced with the transmission in the car.
A leak at the rear of the extension housing where the driveshaft yoke slips in should be obvious too if it leaks at the very back of the extension housing.