First, the car hasn't been driven since 1993 and the brake lines are on the replace list before it hits the road again.
sixstarved - guilty as charged. It's amazing how ignorant I was in '90.
The oversize u-bolts are a leftover from the H-O Racing rear sway bar kit that didn't quite fit (now I wonder if the 3-leaf screwed up the geometry). The nut has a light countersink on the top so a traditional lock washer won't work. I'll think about a jam nut, though.
Bjorn - there's easily three inches of u-bolt threads protruding. The bottom of the bolt is even with the shock mount bottom.
I guess with a recheck of the tightness I won't worry about it. Thanks again.