The 300E 3.07:1 differential is smaller than a 400E and will not fit properly. I am not certain what differentials would be a direct bolt in. MB did put a 3.27:1 rear gear in 1990 300CE's. You would definitely have to swap the differential rear cover, but this MAY be closer to a bolt in. Don't forget about speedo calibration when changing gear ratios. Also, your cruise may act oddly with a different ratio.

Scott P.

1986 560SEC (with 3.27 rear end)
1996 E320
1994 S600