According to this:
W124-320E-2-Getriebe/en
The diff ratio on the e320 (with 5-speed auto) is 3.69 and the 5th gear ratio is 0.75, so I could grab a 3.69 diff from a 240D. But since I like a final gear that is on the taller side (lower cruising rpms), I might go back to my stock 3.07.