Explain don't start? Does the starter crank the engine over and the engine not start or is the starter not turning when you attempt to start the engine? Do you hear any sounds when you try to start the engine, radio off?
If the starter isn't turning the engine over, find the start relay, be sure the transmission is in neutral and the handbrake on, hold a finger on the side on the start relay and have an assistant turn the ignition to the start position. If the start relay is working you should feel and hear a click. If it isn't clicking, connect a jumper cable with clips from the brown wire on the relay to the white/brown wire on the relay. If the engine turns over the starter and start solenoid are good. To test the relay connect the brown wire to (your 73 may have a Yellow/pink wire instead of a red/white wire) w/r or y/p. If the relay is good the starter will operate regardless of the ignition switch position. If everything works to this point there is a problem in the ignition switch, the seat belt sequential system (if you have a yellow/pink wire on the relay) or wiring between the switch and the start relay. You will need a tester with a light or a voltmeter and the wiring diagram to determine where you are losing the start voltage.
Let us know what happens.
Clifton