I would try a big truck repair there is one in Akron Ohio that I would call and I'm sure they would be way cheaper than a classic car repair place. They make any tach drive cables any length,
Stan, did you have any luck with Instrument Exchange? I haven't dealt with them in a lot of years, but back then, there wasn't much they didn't do. They even calibrated speedometers for the city, county, & state.