I would turn the key on and put it in neutral, arc the starter solenoid with a push button bypassing ignition switch. If it starts, the neutral safety switch could be bad. If the battery connection is good the headlights should be bright , eliminating the battery wires. I don't know if they had fusible links in them back then but strange things happen sometimes.

May be the contact key doesn´t connection to the "start" position. I suggest you will conect a cable on the positive of the batery, and with the other wire of the cable, touch the conection of the solenoid of the starter; don´t worry, but be careful, because the engine will turn. If the engine turns, the problem is in the contact key, or in the cable between key-solenoid of the starter. good look! Alphacharly

have you checked the red wire that comes off starter for brake? try jumping large post to small post on starter,if nothing happens there you have a bad connection at battery cables to block or battery.