You need to also have a wire from the Pos. post on the battery to go to the juntion box. Did the 77 Pos. post have the big cable to the starter and a smaller wire? That little wire is a fusable link wire. It gives power to the Juction box, and everything else including the alt. post. So you had it figured out. Just was not wired where all the wire are tucked in hidden from view. Please use that fusable link though it is the electrical overload protection for the truck.