727 Transmission leaking

Did you use a ball pien hammer to dimple the bolt holes outward or toward the bottom ? Then lay the pan flange down on a flat surface to check it ?
Trans pans get beat up and warped. If its done right cork or vulcanized rubber gasket will seal it without added sealant.
Old school method of finding the leak... wash it all down good and use toilet paper to wick the leak. Poke it in around line fittings, selector shaft, dip stick tube, then wrap the pan flange. Come back in a couple hours and find the leak evidence.