Go back to basics first.

You need 4 cables to the battery as a minimum.

1. Thick cable from +ve battery terminal directly to the starter.
2. Thinner cable (6mm) from -ve battery terminal to the alternator output
3. Thick cable from -ve battery terminal directly to a bolt somewhere on the engine on head. From memory, the original V8 earth cable used a bolt at the alternator bracket.
4. Thinner cable (6mm) from -ve battery terminal directly to a good body bolt. I think the original went to a radiator mount bolt or the radiator support panel itself.

The 2 thick (heavy) cables only carry starter motor current, the other 2 carry the current for the other stuff (lights, relays, instruments, heater fan etc.).

As agro71 has said, the coil voltages will vary depending on whether it has points (& whether they're open or closed). If it has electronic ignition &/or a ballast resistor the voltages again will be different.

Dr Terry