The only really sensible change is to locate a five speed transmission and get it installed. They are scarce but still out there.

The only rear end ratio change I would otherwise consider is the 3.47 from any non turbo five speed 123. Your present ratio is 3.69 so the improvement is only 7 percent if I remember. Yet seven percent is still a step in the right direction.

You in my opinion do not have enough power to go to a much lower ratio rear end in my opinion. I have two 240ds by the way. Also you will need the speedometer head from the donar car.

I personally believe that a five speed with a 3.47 rear end would be the best possible combination for a 240d. The slight background noise at 70 that takes a good sterio to overcome varies from car to car. For example one of mine is really noisy at 70 in comparison to the other.

If you like the car basically take the time and effort to get the fuel pressure right in the base of the injection pump. Yours may or may not be where it should be.. Find out. It makes a difference in many ways.