Most of these are driven off the distributor, like on older Vettes. That said, they aren't real reliable and running the cable to the engine from the dash on a mid-engine car makes for a long cable and a finicky setup. I have a Smiths tacho which runs perfectly off my MSD distributor and it's all electronic. And it looks the part. I had the best luck with the Smiths guys in the Midwest, the guys in New York are not real helpful and kind of rude.
If you are determined to do this, I think MSD do make tach-drive distributors, and they could probably give you some suggestions.