long to short tailshaft swap question

Well now that my son and I finally got his 70 Duster project on the road, we are very unhappy with the a833od we put together, basically we built one out of two we had using original parts and re-sealed it but it is noisy, so we have a couple really clean mid eighties truck trans but the tail shafts are too long what is involved in swapping the tail shafts?, I considered doing this back when we put it together but it seemed like too much work has anyone here done this? I am a little more familiar with the internals after having a couple apart now, I really wouldn't want to use any of the gears from the original we have as I am not exactly sure where the noise is stemming from, thanks.