A common problem with the TH350 is for them to leak out the front seal when sitting for a few days or longer. This typically happens on units with a fair amount of mileage on them because of seal wear and hardening.
The convertor slowly drains back into the tranny case and the fluid gets high enough to leak out the front pump seal at the hub of the convertor. If I remember correctly there is an anti drain back check valve built into the front pump to hold the fluid in the convertor but over time it can leak by. Add in a worn and stiff seal and you have the mystery leak. Drive the car every day and the leak seems to go away.
While you are under there check the dipstick fill tube where it goes into the case and the cooling lines at the case fittings. The other two places to look are where the shift arm comes out the side above the pan on the driver's side and the tailshaft seal at the yoke.
A clean piece of cardboard under the tranny can narrow down the source area of the leak.
