I have a 1969 Firebird 400 Convertible and the plunger busted on the near impossible to find Kickdown Switch for my carburetor mounted Turbo 400 transmission. I did find one at Yesteryear Antique Auto Parts in Florida but it was like $400.
I found a work around part that was an inexpensive solution for me. I thought it might help others with in the same spot.
The switch I used is a Standard Brake Light Stoplight Switch Part# SLS-110 - I think it is actually a cruise control as the Kickdown Switch for the T400 needs to be pressed down to engage the passing gear.
I would enclose a picture that shows how it is set up but can't here. I did post pics here but you have to log in.