Lokar makes a kit that will do this job. I'm still looking for an original kickdown switch for mine.

You run 12 Volts to a switch on the carb or under the throttle pedal that closes at nearly wide open throttle. The other side of that switch runs to the solonoid on the side of the TH400. When you are WOT, the solonoid in the TH400 operates. It downshifts the TH400 for you at WOT. You can do this manually with the shifter. Some use to call it "passing gear".