This is fail safe.
Enlarge the cable hole on the throttle bracket to where the 24V cable will fit though it and catch as it should.
Then cut the plastic sheath off the cable along with the clip on end leaving just the cable and about 2" of the rod.
Back to the Throttle Bracket, looking at the arm where the 12V cable should attach is a stud. Cut the stud off, then drill a 3/8" hole in its place. Place a 2.5 " stainless shouldered 3/8" bolt through the hole. Secure it with 2 lock nuts, one on each side. You can see it in the pic.
Drill a hole in the bolt big enough for the pin on the cable to pass though. With the whole assembly on the truck and the pin in the bolt use your hands to pull the pump dead shut and the cable to the closed postinion (pedal off the floor) and lock this position with some vice grips on the radiator side of the bolt. The drill a hole through both the bolt and pin (with a bit smaller than the pin!)
Then cut off the extra of the pin off (that little bit sticking out) and your done.