1955 Plymouth leaky transmission shifter seal

[lionheart228]

I have a 1955 Belvedere with PowerFlite transmission.   

When the car is idle the torque converter drains down into the sump and the ATF eventually leaks out the shifter seal on the side of transmission. 
I have heard this is a known problem.

Where can I get a good seal kit?   I am not sure if good modern seal kits are made. 

We tried a seal kit from Fatsco which didn't last very long.