You can use a variety of aftermarket kick down switches and relays. You don't need to pay for the overly expensive original stuff on ebay. They can be found at Napa and also pick your part.
Fred


1956 3100 Pickup/Red/350/3sp OD/PS/Disc Brakes
1957 Bel Air Sport Coupe/Red/355/TH350/PS/Disc Brakes
2017 Silverado LT Single Cab SB/Black/5.3/6 Speed Trans
1947 Willys CJ2A w/F-Head engine