My truck had an Edelbrock 1401 on it, and no linkage to the kickdown switch.
I bought a kickdown device from Lokar. I haven't installed it, yet. However, I had my transmission serviced, and also had the valve body modified for firmer shifts. They checked the kickdown switch in the tranny, and it is okay. I just need to get busy and get the Lokar piece installed. About $60. Go to
www.lokar.com, for more info, or check with a hotrod shop in your area.
Oh, I've since changed to an Edelbrock 1901 Q-Jet. I still don't have the kickdown linkage hooked up, but the modified tranny will kick down, as determined by a lack of vacuum at this time. The modulator is the smallest canister that would work with my TH400. Only thing I miss, is the kickdown to 1st.