Have finally gotten around to scanning in the schematic for the tachometer adapter circuit. This is essentially the same as the front-end used by Pontiac beginning in 1972.

Pontiac had tachometers available in their cars beginning in 1963 with a dash mounted unit. Visually identical, there were actually two different tach circuits used, a simple passive (and none too acurate) circuit for use with point ignitions, and a more complex transistorized one used with the Delcotronic transistor ignition system. This continued through 1966.

In 1967 Pontiac introduced the hod tach but since the Delcotronic TI was replaced by a CD unit that year, the passive circuit sufficed. No transitorized hood tachs were made by GM.

In 1972, all tachometers used a transisorized circuit to match the general availability of the Unitized Ignition.

In 1973 GM generally converted to the air core tach, a transistorized unit but with a completely differenct circuit

However the early transitor circuit and passive circuits are very similar - the transistorized on is essentially an amplifying front end for the passive circuit. I have had remarkable luck just with constructing that front end and applying it to both TI and HEI ignitions so that the OEM passive tach can be used with these.

Parts are readily available at parts houses - you can even find almost everything at Radio Shack. For safety, use a 1 amp fuse inline with the coil (tach on HEI) wire. Good luck.

Note: for use with a HEI you might want to replace the 3.3k R7 with a 1.8k resistor and move CR1 to the leading side of the C2 junction.

Just as a final note - I have converted hood tachs this way using the stock harness by moving the grey wire (lights) to the accessory terminal on the fuse block and taking the 12v from there. Downside is that the hood tach lights are now on whenever the ignition is and you cannot dim them (but are not that bright to start with).