Be sure to also get the pedal box, front half of drive shaft, flywheel and clutch pipe. The 5 speed has a 1:1 4th gear just like the auto and a 0.8:1 overdrive 5th. The engine will be turning 20% slower on the freeway given that u keep the same diff. The 5 spd is identical betwen W124 and W201. The clutch pedal box will also bolt right in. Not sure about drive shaft length.
Good luck

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CENSORED due to not family friendly words