Dan,
Do you have all the hangers that come with the car?
First one is the transmission connection, then the midway down the driveshaft tunnel, another big one and then the next one just before the big silencer in the rear, and the ladt at the very end before it exits under the bumper. Seems like there is an uneven stress on the rubbers. Make sure the system is level and not stretching the rubbers too much. Make sure you use the heavy duty ones.
There is a rubber bump stop attached to the chassis rail at the last hanger place to prevent the pipes from hitting the chassis. Make sure you have this one.
If the silencer is still hitting the fender lip, then you can bend the silencer upwards a bit(axle side lower).
My 1967 220SL also has the same problem, but it was fixed with heavy duty rubbers, that are very stiff.
If the problem still persists, then you can use a hose clamp and tighten it to the right snugness.
Walter
1967 220Sl-diesel