Thanks for your help NachoRT74! After getting a close look at that old dry plastic bushing it seems to me like this would be a typical problem for ~50 YO cars. The repair was a success and I was able to calibrate the speedo (850 rpm / 51 mph) with a nice firm pressure holding the coil spring in the set position (after the epoxy cured). As you said the trickiest part is holding the bushing in place. I couldn't quite visualize how to use a fork so I made a simple sheet metal clip to hold it while I applied the epoxy.