Wiring Aftermarket Gauges

You probably already know your 75 model has a white fuse box with about twice as many fuses as the earlier black 5 circuit fuse box shown above.
If this is a gauge pod that mounts under the dash, like under the radio maybe, All the power you need is right there at the radio and already fused. The orange wire for lighting can be found at the lamp in the top of the radio, or at the ashtray lamp (if so equipped). This allows the instruments to brighten and dim like all other OEM illumination.
Without a 75 manual to reference, I'm going to say the power connector to your radio has a red wire and a white wire. Both are switched hot (white is just extension of same fused circuit. I forget if it went to turn or reverse lamps).
I think adding fuses and relays for a few gauges is overkill. Just properly tap the power sources and install male/female connectors so they can be disconnected if needed.