[82 Silverado 4X]

Mitch,
I can't help on which line goes where on the TH400, but my opinion would be that it should flow from the trans to the radiator and then into the tranny cooler and then back to the tranny.

Doing it this way, it allows the fluid coming out of a radiator which has coolant that normally runs at 180-210 degrees to further cool the fluid as it passes through the tranny cooler before returning to the tranny.