Shaun,
I have a 72, not a 69 like you're building, so I'm not the one to ask about a tach in a 69, but you may find it in the below link. As you can see, the standard SS gauges did not include a tach, but was an option like you said. Not to mislead you about my gauges, they're not SS gauges, I have two mounted to the column, and two under the dash. I plan to replace with SS gauges in the future.
http://chevellestuff.net/faq/gauges.htm
Gary