Easy to replace front seal on 904?

Hey everyone-
Need some advice from those who know transmissions more than me. We are going to pull the 318 on my 73 Dart for a rebuild. I ordered a front pump seal kit- gasket, O-ring, and pump seal from something I read. Is this easy to do with the trans in the car or does the tranny need to come out? Its not leaking or having any problems now so maybe if its too much trouble just leave it alone?
Its a 904.

Thanks