Wiring Aftermarket Gauges

I am installing an Autogauge 2 5/8"gauge pod (water temp, oil pressure and voltmeter) and tachometer in my '75 318 Duster as part of my "Day 2" look build. I've the mechanical part done, now starting on the electrical hookups. Does anyone have suggestions on what wires to splice into for the lighting and switched power? The fuse panel isn't marked and I don't want to randomly start splicing into the almost 50-year-old wiring harness. Thanks!