Usually, people complain about 400 not downshifting at speed.
Make sure the trans is full of fluid, check it with the engine hot, idling in park.
Cab you duplicate the problem? Or was it a one time thing?
The kickdown solenoid is either on or off. It should energize about the same time as WOT, even if the switch was adjusted to engage a little sooner the actual upshift/downshift points are controlled by the valve body.
If it really bothers you, and you want to fix it, I’d start with inspecting the govenor. Make sure the teeth aren’t chewed up, make sure the bore the governor spins in isn’t scratched up. Make sure the weights move freely. If that all checks out, reinstall the governor, and try to install the governor cover without the gasket. Ideally, the cover should just barely fit. Adding the gasket at that point should provide just enough clearance. If it’s still sloppy, lightly dimple the center of the cover to tighten the clearance.
After all that, if it still has a overactive passing gear, it’s time to remove the valve body.
