Jim, thats a NO-GO for 1968 350/400 birds with aftermarket second gen-GM HEI.

There are only two wires connected to the starter for us: The main POS cable from the battery, and the "S" connector on the solenoid. There are not three connections as is normal and indicated in the 1968 diagrams. Only two, been driving just fine this way for many years.

Connecting the third wire (to the solenoid "D" terminal I believe) causes the starter to re-engage and the POS connector on the battery to overheat and melt down. I've seen it myself first hand. Maybe its just my particular GM HEI distro.