Swap manual /6 with auto /6 - what do I need to do?

I believe you will be ok if you stay with a engine from a 72 or older, I think it was in 73 they changed the flange on the crank to be a little larger. It's been a few years since I did it in my Dart. Oh and one other thing, on the newer models the pan is different so you might have ot put your old one on the new motor.