I was unable to access your google image but in a 66 there should be two round metal knobs on the floor, the gas pedal has pockets for these to sit in. LMC truck does sell these (
https://www.lmctruck.com/1960-66-che...r-rubber-parts) but there are other sites that should carry them too (i.e. Brothers Trucks, Classic Car industries, GMC Pauls etc.)