You'll also need the 12 volt type foot start switch with the small side terminal for the resistor bypass wire. Rock also shows a listing for that item. Search their website for the 1957 C-10 with a 235 for a "starter solenoid". That's not the correct name, but that's how it's listed.
I lied- - - - -they corrected the listing!
https://www.rockauto.com/en/catalog/chevrolet,1957,truck,3.8l+235cid+l6,1326091,electrical-switch+&+relay,ignition+starter+switch,4700
Jerry