I can't be sure but I think I remember that my 410s would change down to first if the floor selector was used at around 25MPH. I will have to check but haven't got the car at home at the moment.
It looks as though it is possible to convert early Bristol Torqueflites to part throttle kickdown if you can find the kit:
727 3-2 Kick Down Module (1966-69) - A&A Transmissions