You can get a switch from Rock Auto for under $4.00.
Airtex/Wells ADV 4C2, 1984 Chevrolet El Camino stoplight switch.
Pick up 2ea 1/2" NF jamb nuts for the switch. Fabricate a bracket to mount on the front of the carb to actuate the switch at WOT. Wire it up and you are good to go.
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