Does anyone have a Bilstein shock that they can measure, or does anyone know, off hand, what the diameter of the fat part of the shock is?

My rear trailing arm has rust at the mounts for the shocks and I would like to reinforce that area with a large diameter washer if possible. So I need to know how large the inside diameter needs to be to see if this is even possible. If anyone knows the diameter of the hole in the trailing arm, that would be great too. I would measure all this stuff, but I didn't drive the car to work this morning.

Has anyone done anything like this before?

Terrible idea?

The car in question is a 1985 300CD