1968 Dart ... Need wiring help

I just don't like "one wire" they are not necessarily "better" than multi wire and are harder to "kill" for things like a battery disconnect.

You definately need to bypass the ammeter (AKA Crackedback and at least read the MAD article.)

He also sells headlight relay kits

If nothing else one thing that you must be careful of with "one wire" setups is to run a MUCH larger charge wire, because the voltage sensing is on that wire. So voltage drop becomes more critical. For anthing more than 60-70A I would run no6. Up to about that no8