The A.I.M. (M40, A9) shows the kickdown switch bracket on the driver's side rear carb stud on all big-blocks, and in (J50, A4) shows the power brake pipe & support bracket in the same location for all big-blocks, but there's no illustration reference in either section that shows both parts used together. Since the switch location is important and its bracket is flat, and the power brake pipe & support location isn't critical and it has a raised rib on it, logic would say the power brake pipe & support went on top of the kickdown switch bracket.
Maybe someone with a power brake big-block automatic can confirm? KurtS?