73 valiant no spark from coil

My best guess. The starter along with a few other draws are using up your available voltage. When the starter draw stops, you get a spark. There is a huge wire supplying voltage to the starter. Everything else is attempting to operate on a much smaller wire.
Further reduce that supply at ignition switch contacts, various harness connections, etc.. and get the condition you have.
Couple places to look at... Look for a white engine harness connector. It has the alternator wires and others going through it. Its crap !
Look for a small white ceramic block with 2 wires. That would be a electric choke assist. Disconnect it. It is also crap. Neither of these were ever really needed. Chokes worked without the electric assist. Engine harness connector only eased assembly.
Good luck.