Not sure if anyone has messed with the wiring on your Pantera or changed the ignition system to HEI but here's my hypothesis.
When your ignition switch is in the start position, there should be a wire from the starter relay on the firewall in the engine bay behind the passenger seat that goes to the positive side of the coil and provides 12v to give the ignition extra power to start.
When you let the key return from start to run, the coil should be getting less voltage (around 7 volts). You should be able to get in and see if you're getting voltage by using a test light on the coil with the key in different positions.
I'm guessing you aren't getting voltage at the coil in the run position.
Possible causes could be
1 - Blown ignition fuse -- check all the fuse connections and power to both sides of the fuse in the panel on the passenger's kick panel for a pre-L
2 - Bad ballast resistor - Check voltage coming to balast resistor and voltage leaving ballast resistor (located next to starter relay an firewall
3 - Bad ignition switch - Drop the steering column and there is a four terminal connector that you should be able to test with your volt meter or test light and confirm that your switch is sending power when it's in the run position.
https://pantera.infopop.cc/topic/ingnition-switch
Hope this helps you zero in on the issue