73 valiant no spark from coil

Voltage is WAY low. Either you have serious drop in the harness, or the battery is just plain dead

What does the battery voltage read when cranking?

Here's what you are up against, and why you need to crank using the key when testing for "real world" conditions:

The ignition switch has SEVERAL separate switches

The IGNITION RUN (IGN1) is hot ONLY in "run" not start

The BYPASS circuit, (IGN2) is hot in start, and is the only source of ignition to the system

The "path" that is the circuit path both these follow is:

From battery........fuse link.........through bulkhead connector..........to ammeter........through ammeter..........to ignition switch connector...........through switch........back out switch connector.........back out bulkhead connector..........to ignition system

If you have one or more bad connections in the above points, you will have VOLTAGE DROP to the ignition system.

Start by cleaning the battery terminals.

Measure battery cranking voltage right at the battery terminals. If you have less than 10V the battery is dead or going bad. A good battery which is charged will crank at 11V or so.