I've been looking through my old pics and such of the truck and things I did to make it work. This is one of them. This is likely from 2006 or 2007.
That truck never downshifted and irritated me greatly on the freeway. after this fix, 20 years later still works like a charm.
Have you verified that the carburetor throttle plates open 100% whenever the accelerator pedal is depressed to the floor?
In many instances in which the accelerator pedal lever arm didn't contact or sufficiently depress the detent switch, I found that the lever arm was distorted from its original configuration, preventing WOT. Sometimes, the problem was introduced when a truck that was originally delivered from the factory with rubber floor mats was subsequently fitted with carpet and pad, effectively decreasing accelerator pedal clearance with the floorboard, hence pedal travel before contacting the floorboard. The decreased pedal travel limited both throttle deflection and detent function. In both scenarios, bending the arm into its original geometry restored both THM 400 detent and WOT. It could be worth a look.