Lets step back to look at the basic wiring concept.
There's a welded splice which joined the alternator and the battery with the main feeds to the key switch, lights, fusebox hot, and sometimes horn. Regardless of where power came from, battery or alternator, it went where needed from the main splice.
The fuse box has 2 - 3 fuses that get power only when the key is on, and 2-3 that are always hot.
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Above I wrote "a welded splice which joined" in past tense. That's because '75 is a little different.
Also if the car had rear window defrost its again a little different.