A mechanical speedometer would be very reasonably price, Autometer and Stewart Warner have some that would fit the era and them of the build. My GM parts guy at a dealership here in town has a book with a nice little chart in it for determining the right speedo gear, takes in the tire diameter, rear gear, and final drive ratio of the tranny. What kind of trans is in the car, anyway? The electronic speedometers are neat, simple to calibrate, but quite a bit more spendy.....