Don't use washers to increase the spacing - Mercedes use what is known as "hubcentric" wheels - meaning that the circular center of the wheels mates with a corresponding circle on the hub. It is this precise matching that bears the weight of the car on the wheels. If you use washers - the wheels may not be seated on the hub. Mercedes wheels bolts aren't meant to support the weight of the car - but rather just to keep the wheel in place. Buy yourself the correct length lugnuts. Your wheels could suffer from balance problems to much worse with washers in place.

Troy
1995 E420 84k