I just ordered the motor mounts for my 300E w/130K mi and will be doing the job next week. I will measure before and afer and let you know if no one else has posted a spec. I'm also replacing that pully damper as it is squeaking. In talking to other owners, the motor mounts and the damper seem to be common replacements at 100,000 to 125,000 mi (also radiators and water pumps).
I noticed you are in the bay area. I'm in Novato and will give you my phone # if you e-mail me so we can agree on where to measure the mounts from.