No elec. power to my 66 Dart

I agree with most of the others here, you lost your main 12v source going into the firewall. It's open somewhere in the series circuit, most likely at the ammeter but it very well could be the fusible link connected to the red wire at the firewall or the connectors on the same wire. The wiring goes in via the fat red wire and comes out via the fat black wire directly to the alternator "bat" terminal. Remember, you're looking for an open in the circuit, not a short.