On AD trucks, the anchor bracket for the springs is riveted to the frame. That Z shaped bracket would be pretty easy to fabricate if someone could share the dimensions with you.
The hook end of your extension wire would go on the brake pedal and then the spring would hook in the looped end.
You might PM Homer52. Dave has both a 1946 and a 1941 1/2 ton pickup and might be able to give you some more info. Although his 1946 is on an S10 chassis, the 1941 is stock. He might also have another truck of that era tucked away under a tarp.
