I just replaced a Turbo 350 in my 1937 Dodge pickup, it shifts correctly but will not down shift at WOT. I've checked the TV adjustment and replaced the modulator valve but it will not downshift. The vacuum line is connected with no leaks and the engine has 16-17 of vacuum at idle. The transmission I replaced would down shift at WOT so the new tranny should work.

If the cable is adjusted correctly and the modulator is working what else could cause it not to downshift?