74 dart has electrical power but no start

Yup. transmission? Auto? Do still have the clutch start safety switch? (4 speed?)

You should be able to narrow it down with a few simple tests. Go around front and jumper the only exposed terminals on the starter relay. If it cranks, it's in the neutral safety/ harness/ ignition switch end. If it does not, it's a starter / cable problem

If this cranks, take a test lamp to the two "push on" connectors on the start relay. Pick one and unhook it. Probe each connector, have a friend twist the key to start. One of them will "go hot." Hook that one to the relay, and GROUND THE OTHER terminal. It should crank.

If so, it's in the wire/ to the / and the / transmission neutral safety switch.

If it's a 4 speed, it's the wire/ to the / and the clutch start safety switch.