I dropped a 5 Cylinder Turbodiesel into my W123 (240D) with a manual transmission. The stock gearing spins the motor at an unnerving RPM; my solution was to bolt the differential out of a non-turbo W123 (300D) wagon - this was a direct bolt in replacement. I don't recall the ratio but it lowered the RPMs 11% or 12% (off the top of my head - again) the result was a blast to drive since nobody ever expected to see tire smoke from the diminutive 240. The swap required me to cut about 4" out of the front driveshaft and shift linkage and modify the transmission mount somewhat but it was WELL WORTH IT!