904 won't go into passing gear...

Well, you didn't say one way or another, but if it STILL won't kick down, get a buddy to eyeball the carb throttle blades, with your accelerator pedal on the floor (engine off, naturally) and if the throttles are in fact, going wide open, and simultaneously, the throttle-pressure linkage is pulled all the way forward (you might have to remove the linkage arm from the lever on the transmission to ascertain this), then I don't know what to tell you... but, I'm betting that if it STILL doesn't shift down into "passing gear" at speeds below about 60, then there reamains a problem with throttle (carb) linkage or throttle pressure (tranny) adjustment. Not too much can go wrong with a T-Flite that will prevent it from "kicking down."
Keep us posted.