I'm not positive on the th400, but most automatic output shafts have a "blind spline", meaning 2 splines joined together at the end. If you look at the female splines of the transmission slip yoke, you may see what appears to me a missing tooth. This would line up with the blind spine of the transmission output shaft.
This is the method used for ensuring the driveshaft goes on the same way, or as Jeff says "in-phase".
