1970 Dart starting issues- key will not crank now

Let's sum this up, possible problems No particular order

1....Low "yellow" start wire voltage with key in crank. Bad ignition switch, bad switch connector, bad bulkhead connector

2....poor ground at relay neutral safety switch. Bad safety switch, bad switch adjustment (transmission linkage) or bad wire connections

3....Bad starter relay. Contacts are corroded/ burnt, or junk/ corrosion in relay prevents proper movement, or some problem with the relay magnetic coil

4....Sometimes the wire from the start relay 'square' connection goes bad down near the starter solenoid. But you say you can jumper the solenoid and it cranks OK. So that wire should be OK

These are a simple circuit and work well. We can walk you through this