I was looking at the wiring diagram for my brother's 74 Duster. The starter motor and starter relay are fed directly off the battery. Since you have no power to anything, and the starter motor does not even attempt to engage when you turn the key, and since the battery has tested to be a good battery and it has a charge, I believe that your problem lies in the battery cables or in the power distribution from the ignition switch to the starting circuit.

I forgot to mention the fusible link in my previous suggestion.

Try jumping the two terminals on the starter motor. If the starter motor engages the engine and spins, then you know you are getting power from the battery to the starter motor and that the battery cables are OK. If that test checks out OK, then you need to get a test light and just start probing the (+) feed starting at the battery until you find the point where you are losing power. Probe the (+) feed on both sides of the fusible link and on both sides of the bulkhead connector. Probe the feed on both sides of the ammeter.

You can TEMPORARILY AND FOR TESTING PURPOSES ONLY remove the ammeter from the circuit but removing the wires from the back of the gauge and connecting them together using a small bolt and nut through the eyelets on the wires. Be sure to securely wrap this connection with electrical tape to prevent any accidental grounding of the terminals against a ground on the dash frame. You must properly reconnect these terminals to the ammeter once you have found the break in your power supply and have repaired it.

From the ammeter move forward to fuse box, the ignition switch, and the headlight switch.

The break in your power supply should be at or ahead of the ammeter since you are losing power to the entire vehicle.