That's a aftermarket tach. The factory tachs are engine specific, meaning there was a different tach for 4, 6, and 8 cylinder engines. And they are a current triggered tach wired in-series with the ignition system. Yours isn't.Just added a tach to my DUI setup as well. This is the wiring diagram supplied. I used the tach output from the dizzy and sourced new keyed power and ground.
Unfortunately, I can't answer Clark's question as I've never messed with a HEI system on a Jeep. Sorry.