67 Barracuda dash wireing

The hefty black wire is on one side of the ALT' gauge and is hot. It's black because this side of the gauge is where current does eventually reach grounds after going through nearly everything else.
If you want to add a chassis ground wire, the best location is the lower screw at center pod. Approx 4 inches below, add a male female spade connection for service disconnect. Then enough wire to attach ground behind left kick panel.
Although all 6 of the little screws that hold the circuit boards on the housing are chassis ground points but.... those screws aren't long enough and the pot metal threads aren't strong enough for adding/securing a ring terminal and toothed washer. Just carefully remove them, clean the copper pad where they contact and clean their kept washer then carefully reinstall.
There's yet another reason why we shouldn't add a bunch of wires all over the back of this panel. Enough said for now.