The wire going to Vile's pic would lead to the kickdown switch mounted behind the gas pedal, so when the pedal is dressed all the way than the switch will activate the kickdown in the tranny. The other wire from the kickdown will then go to a hot lead, can't remember if it is keyed or always hot.
First pic = kickdown switch
Second pic = where it the switch goes in the cab
Third pic= where the wire attaches on the 400 (bad pic, sorry)
I to have a 77 stepside now with a 350 and a 400, really can't stand the 3 gears though, you may plan on having to change your shift points and get a new speedo cable.
Good Luck
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