I have a friend who is putting a 302SB in a GMC pick up and is having problems getting the tranny and engine together. He can get it together but the back seal leaks ,everytime. Anybody have any idea what the cause could be? I will post back with more info on year and model of tranny.

i would guess it has to do with the sealing surface...

take it apart, clean the surface and put ina new seal. make sure its the right one ya got