74 Dart won't start - ignition switch - maybe starter relay?

Other than the interlock, Mopar starter relays are easy. The big stud is always hot battery, and one of the relay contacts. The other contact (load) is the "square screw" which goes down to the starter solenoid

The coil is the two push on flag terminals, one is grounded by the NSS and the other gets power in "crank" from the igntiion switch

Might note that Mopar IGN voltage is a "trick."

IGN1 or "ignition run" is ONLY hot in "run." It goes "cold" in "start." Ignition start voltage is supplied by the bypass circuit (usually brown) IGN2. It comes from a separate contact on the ignition switch and goes to the coil side of the ballast resistor. So it supplies full battery during cranking. THE BALLAST works "backwards" during cranking to supply power to the ignition box.