73 dart not cranking

OK check "coming out" of the ignition switch for power at the yellow start wire

73? Can't remember "might" have the dreaded seat belt interlock, if so the yellow start wire goes through the reset box contacts:

http://www.forabodiesonly.com/mopar/showthread.php?t=292463

http://www.forabodiesonly.com/mopar/showthread.php?p=1969624169

http://www.forabodiesonly.com/mopar/showthread.php?t=14304

If you DO have the interlock, find the two yellow / yellow tracer wires in the reset connector and hook them permanently

"Might be" neutral safety or clutch switch going bad. Identify the NSS wire on the start relay, comes of one of the two small "push on" terminals goes down the firewall to the transmission.

You can disconnect that and clip that relay terminal to ground to test. "BE CAREFUL" it will now start in any gear

"Might" be the starter relay going bad. Can you hear anything click? (quietly) when it won't crank?

Relay clicking "quietly" and no crank, might be bad, otherwise is bad wire to starter or bad starter

One loud "clank" is starter solenoid pulling in, if it's that, then bad battery cable to starter, or bad starter.