Not sure if you're doing a restoration or this is for function, but I found my T400 downshifts fine w/o the switch.

On another note, I noticed some of the pictures show a single return spring. For safety sake I always run 2 springs, which If I recall this is the stock configuration. The Chevy return spring pulls on the top of the bell-crank and is attached forward. Whereas the Pontiac springs pull on the bottom and attaches to the accelerator cable bracket.