There needs to be a switch attached to the throttle linkage so that it closes when the linkage reaches full-throttle.
If I remember correctly, 12v switched by ignition goes to one side of switch; wire from other side of switch goes to the single connector on trans.
I have seen switches placed on a home-made bracket mounted to a carb stud & activated by the linkage. '67 & later trucks factory equipped with a 400 have a switch under the dashboard that is activated by the linkage.
An alternative is a street rod part that connects a cable to the throttle linkage; the other end goes to a remote-mounted switch that sends a signal to the solenoid inside the trans. Check a magazine like Street Rodder for ads, or consult a street rod store. Lokar or Gennie Shifter seem like the brand name.
