You can swap out your earlier mechanical tach with a later electronic one from a '75 through '77 C3 ... you can also put your old face on the newer tach if your gage colors do not match. There are others here who've already done this. I will do this in my '71 ... I got a good used '75 C3 tach from ebay for about $50. And as Bullit mentioned, there is a new HEI clone offered (but with mechanical tach drive) ... but I think it's about $400 -$500 at Mid America.
I know I am not answering you direct question here, but why not look into a Pertronix conversion to your old points? It will allow you to go to fully electronic ignition, use your same distributor that you have now, and the tach will keep on working just like it does now.
The swap will take a Saturday afternoon, require some basic tools and a bench vise, and cost under $100.
You can buy them at Summit racing and many other places. I installed one a couple of months ago along with Garin1 on his 68 convertible. Pretty straight forward operation, and even if you are going NCRS with the car, the experts will have a hard time telling you are sporting dependable electronic spark.
I'm with you, Chris. I'll install a Pertronix soon, as it already is sitting on my parts shelf. Besides, a friend gave me a Blaster 2 coil awhile back. I want to acquire some more parts though, before I tackle the removal of the intake. My problem right now is deciding whether I want to change the camshaft or not. Decisions, decisions.... --Bullitt
we did the same thing on my buddy's 68 ragtop last year, bought the pertronix unit and a high volt coil from summit and installed it the same day (we live about 70 miles from the summit h.q. in ohio), my buddy loves it ! by far the easiest fix for your dilema.