I don't know if they make them for that transmission but if you can find or make a set of reinforcement rails like Ford used on some 302 oil pans it might help.
Example
Random thoughts:
Aftermarket pans should have thicker pan flanges on them for better sealing.
Check the transmission bolts and bolt holes if a bolt is too long or if the hole is dirty, damaged or not tapped all the way the fastener will bottom out (tighen up) without sealing the pan all the way. It would appear tight but not sealing all the way.
There could be porosity or small crack in the transmission housing casting allowing a small amount of fluid to leak out.
The transmission pan and/or mating surface might not be flat.
Is there a torqueing pattern or sequence that has to be followed?
Could the leak be coming from elsewhere on the transmission case and just collecting there?
Are there any humps or high spots around the holes in the pan?
Transmission mount and crossmember in good shape and bolts tight?
Inadequate gasket? Try a Ford Motorcraft OEM gasket for it.
Bolts backing off? Clean bolt holes and bolts with solvent before reinstalling. Use a light-duty threadlocker if needed.