You may be reading the voltage from the starter cable when you check across the two terminals.
Remove the heavy purple wire from the starter solenoid S terminal and connect a voltmeter from the wire to a known ground or the negative terminal of the battery. It should show zero volts with the key off and battery voltage [+12 V] with the key in the start position. If you have no voltage at the purple wire with the key on start you'll have to backtrack that circuit.
You can check if you have 12 volts coming out of the S terminal of the ignition switch. The start connector of the starter will be marked S or START or SOL for starter solenoid. Juice flows from the switch to the neutral safety switch through a purple and white wire and from the safety switch to the firewall connector through a purple wire, then from the firewall connector block to the starter through a purple wire. I thought the neutral safety switch was on the shifter of the Muncie four speeds but don't know for sure. There are column and floor shifters, auto and standards so there could be various ways of wiring. It could be a clutch switch as well, my GTO was that way. If you have a Tremec five speed overdrive the neutral safety switch is internal, the start circuit is connected to the two terminals of the switch on the upper left side of the transmission tailhousing. It's a normally open switch that is only closed when the transmission is in neutral to prevent the car being started in gear.