Thanks for the quick response. I had a feeling it was probably the speedo unit and found one in a wreck-yard for about $30. I was waiting to send for the used unit until I had time to further investigate. Seems logical that I pull the adapter and count the teeth before proceeding, though. The car is so original in every respect that I seriously doubt anyone ever messed with the drivetrain.

The only odd thing I've found regarding the originality of the car is that the code stamped on the block XI indicates a 326 with a 2 barrel carb. This car has a 4-barrel Carter carb on it. Maybe I misunderstood some code deciphering somewhere but I'm pretty sure I double-checked it and the numbers I pulled from the car indicated a 326/2-barrel w/ California emissions. Other than that, the car appears to be very original.