Is tach wiring in standard rally gauge harness?

Options for this gauge pod were a blank, a tach, a clock, a performance indicator (vacuum gauge). So tach is accessory. Wires probably not in your harness. Some Formula S B'cudas and some 340 Demons, typically manual 4 speed, did have these wires in the car built with the tach option.
These wires attached to the gauge itself. 2 studs on the gauge, no markings but one shorter than the other. Short = sender or signal. The longer gets 12 volts from accessory fuse. Your fuse box may be missing fuse and a clip for 1 end of that fuse.