Not really a torque spec for that. Just get it tight and you be ok. Mark the shaft for return install so the balance stays true to factory. Also, you might as well do both front and back seals as well. It is common for the seals to go out because of a bad center support bearing. You should replace that or it will knock out seal again sooner then you think.
They tend to sag in the carrier because they just get old. They are pretty cheap and I think around 30-40 bucks delivered.