I'm currently updating a 1940 Ford to 12 volt, negative ground.
At this point power hasn't been turned on yet. But one thing no one has mentioned are the books available. I got one from Speedway that has helped. Two points: there is a problem with wiring in an alternator. Depending on how it's done, once you start the engine, it may not shut off with the key. You need either a diode, a relay, or a newer style of ignition switch.
Second point, with an alternator and a wide belt pulley on an older application, I needed a longer than stock fan belt. I learned it is a 'B' or 'BX' series v-belt. Stock is a B51, I used a BX53 (2 inches longer) could have used BX52.