G20 wheels will be the same as C10. As to the Impala SS wheels, they have the same backspace as the truck wheel. They also will not fit onto the truck hubs (although the truck wheels will fit the Impala, that's what GM used for the police car package).
Going aftermarket will be the best bet. Most wheel makers (Weld, American Racing, Wheel Vintiques, Early Wheel Company, Stockton) will make you a set of wheels in any offset/backspace you want.
