One question leads to several questions.
The original tachometer option was mostly used on big trucks.
There are reproduction tachometers (V8 only) available at classic Chevrolet truck websites.
GM part numbers can be found at:
Parts and Accessories Catalog P&A 31 May 1966
Section 9.750 Tachometer Head-Head Assembly-Harness Assembly and Parts
Check the following yourself in case there is an error:
GM #3852111 for 6-cyl engines.
GM #3839947 for V8 engines without transistor ignition.
GM #3885480 for V8 engines with transistor ignition.
Reproduction tachometer - Ignition contacts and/or HEI http://67-72chevytrucks.com/vboard/showthread.php?t=660292
1960-62 Tachometers required transmitters http://67-72chevytrucks.com/vboard/showthread.php?p=8258575
Tachometer adapter, tach adapthttp://67-72chevytrucks.com/vboard/showthread.php?t=536616
I'm signing off on this topic.