Vehicles originally equipped with a TH-400 have a kickdown switch that is activated by the accelerator pedal. There were two types of switches: one located alongside the carburetor and one located just above the accelerator pedal.
You'll need to retrofit one of these in order to make the kickdown functional.
BTW, all this is assuming that you have a stock-style, automatic shifting TH-400 and not a full-manual/trans-brake.