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Take a sec please and fill out your profile so we know things like what year your Vette is.
Also, can you explain better what you're asking? Are you meaning someone replaced your tach drive distributor with an HEI that uses a wire for an electric tach, yet you still have the mechanical one?
The only option I know of is to either convert to an electric tach, or replace that HEI with an HEI modified with a cable drive.
I'm not even sure you can still get them, but they were available and pricey.
I just replaced the distributor in my 350 '69 - it's also got a mechanical tach. I got the distributor from Summit racing, fits perfectly, works extremely well - and it's an HEI. Just call them up, tell them you want a mechanically driven distributor for your tach, and they'll set you up. I think mine was around $250.
I got a factory electronic tach from a later model C3. I think it's a '76. Fits right in, matches well. But most of all it works just fine. I would do it again.