Re: 351c odd problem. Engine only starts when key goes back from "start" to "run"
Just check the wire for clean contacts, check your ignition switch plug for good clean contacts, make sure that there is 12V (well it's actually only what the battery has left available while cranking, normally the 10.9 to 11.5V range) to the coil while cranking.
Take off the batt cable, put a multimeter to the coil + and to the starter side prong. Then hit the key to the Crank position. If you OHM out then that wire is doing it's job, and you should then look into your ignition switch.
It could also come to fact that my above explanation does not apply to your 72, and there is only one terminal on the soleonoid, and your coil get's batt voltage from the ignition switch only...
I hope that the test works for you!