I would most definitely swap the rear differential out.
I just completed the same conversion a week ago, transplanting a 617.952 into a 1981 240d. I did drive it with the 240 3.88 (I think) gears before I put in the diff from the 300 series. The car screamed at 75 - not good for the motors long life IMHO.
I switched out the diffs to the 3.08 that was in the 300, and what a difference!
Good luck,
Adam