Ballast resistor

What is the "mains switch"? I can't make out anything in your photos. There is only the key switch in the factory wiring. Maybe that is a "battery isolation" switch added by a prior owner.

I wouldn't fool with the old Mopar EI in a 67 Barracuda since it wasn't factory anyway. However, sounds like you have the connector for it in your (expensive) YearOne engine harness, so maybe. Otherwise, a GM 8-pin HEI setup is simpler, cheaper, and better.

As said, you will need a ballast resistor. I would wire for the single ballast and get a 4-pin ECU. Most new 5-pin ECU's today are actually 4-pin type with a dummy pin to not worry buyers. The ballast resistor should be 0.5 ohm cold. Get one for electronic ignition, look up 1975 to be safe.