I also have the "missing" neutral safety switch in the new M&H 4 speed harness, I jumpered the two 10GA purple wires in the dash harness to get the car to turn over.
I do not recall any references to such a switch in the service manual for 1968, but that would be a great place to insert a starter kill switch! The factory harness did not have this circuit, the purple went straight to the bulkhead connector from the ignition switch IIRC.
Vikki
1969 Goldenrod Yellow / black 400 convertible numbers matching