If your engine doesn't lend itself to driving a mechanical tach from the oil pump or distributor, then this sealed direct-drive unit may be the answer. It drives your cable from the camshaft.
To install, bore a 15/16 inch hole in the cam cover in line with the cam. Drill a 9/32 inch hole in the cam or timing gear* and install the (included) hardened drive bushing. A short flex cable with square ends (also included) transmits the rotation of the cam to your tach cable while allowing a little misalignment. The shaft is sealed, so you don't need to worry about oil leakage. Four beveled washers on the housing can be rotated to allow up to 6 degrees of casting draft or contour on the cover.
The output is a 7/8-18 thread with .187" tang drive (use with cable part number 1385 for Jones tachs). Since Jones tachs are not direction sensitive you can drive off the front or rear of the cam. Because the camshaft is driven at half the speed of the crankshaft, this drive should be used with a 2:1 tachometer.
* This hole size provides a proper press fit in cast iron or steel parts. Pressing the bushing into harder materials is not recommended. If you are using a hardened bolt, you will need to broach a 0.188" square hole in the bolt head.