1974 dart sport 360 won’t start with key but will start from starter relay

If that does not do it.......

Get yourself a 74 service manual. In leu of that I created this reduced size wiring section of the manual, but I recently discovered it might have a flaw. But it is fairly complete

https://www.forabodiesonly.com/mopar/attachments/74vlwirings-pdf.1715779008/

For everything EXCEPT wiring, go over to MyMopar and download the 73 service manual.
Service Manuals – MyMopar
Also there, download the two page aftermarket wiring diagram for 74. I do not believe, however, that it shows the seat belt interlock stuff. Those diagrams tho, can be easier to follow

Wiring – MyMopar

The electrical path for the starter relay is follows:

From the key, in "start" the YELLOW wire goes through the bulkhead connector, through the seat belt interlock, and over to one of the "push on" flag terminals of the magnetic coil of the starter relay. Through the coil, and out on (if automatic) wire goes down the firewall, down to the center terminal of the transmission neutral safety switch which GROUNDS in neutral or park. If a stick car, the wire is fed back into the interior through a single wire grommet and connects to the clutch pedal switch. So that path pulls in the relay

The contacts of the relay are the "big stud" which is ALWAYS hot and connects to the battery. When the relay closes it sends power down the no12 or no10 wire connected to the "big square" screw terminal and down to the starter solenoid, pulling in the starter

Once you "punch" the reset button, and if it still will not crank, put your test lamp or meter on the yellow wire at the starter relay and see if you get power with key in "start." If so, move to the opposite "push on" terminal and see if there is power there. If so, the neutral switch is NOT grounding. While holding the key to "start" wiggle the shifter through neutral and back to park, listen for a click or other "try" for it to operate.

If that shows OK and or if you can hear the start relay "click" then the contacts are bad, since you already jumped that point (right) and got the starter to crank?