Also if it may help, below is a snapshot of the installation instructions page from a 1957 Chevy Car and Truck Accessory Installation Reference Manual. The template appears to be the same as for the 1956 and think the fender didn't change between 1956/57, at least in that particular section. There is a second page to the instructions with another diagram for routing of the antenna cable into the cab. Let me know if you need that and I will post it later.
