You must have had some rewiring done to the starter in the past. 68 should have a heavy gauge purple wire going to the S terminal from the ignition switch, and a yellow wire going from the R terminal to the coil. If you still have a points ignition you will have to install a diode between the coil and the S terminal at the starter. That will give you full 12 volts to the coil when starting but stop voltage from the coil to the starter when the engine is running. Or you can hook up a ford style starter solenoid, the better of the two options. But as stated above, if you have an HEI the yellow wire should have been removed when the conversion was done.
That's the way I see it.