You only need to drill out the old rivet from the arm. That will leave you with a hole. Don't use the new rivet -- remove it from the sliding jaw and replace it with a bolt. The bolt should be in the same position (end sticking out of the plastic jaw) as the rivet was. See what I mean? Also make sure the bolt is long enough to stick out of the jaw, go through both washers and the arm, and have two nuts on the end!
As far as the nuts, screw one on tight by hand and snug it up with a crescent wrench. Screw the second one on by hand and tighten against the first using the wrench. When you get to the hardware store to buy the bolts & nuts, try this with the two nuts and you'll see how easy it is to make a "double-nut" that doesn't move or unscrew itself.
I'll be checking back here pretty much daily, even through the weekend. Let me know on mercedesshop or via email if you run into trouble.
Neal Bhatt - email@example.com
'83 300SD 193k