You can wire it up not needing engagement cable or foot switch. My installation in 63 C10 went like this:
Used side plate (4-bolt) and shift arms off original three speed. This allowed the use of stock shift linkage arms.
Wired with toggle cut off / cut on switch for solenoid. Now when I want overdrive, flip the switch to on, accelerate to about 30MPH, let off gas and she shifts into OD. Gotta Love It.
Wiring diagram attached:
OD Wiring Diagram.pdf