Hello.

you could get power steering on a car that also had the GT package, as long as it was an 'A' code car. The K codes all had manual steering in 65/66. The purpose of the quicker steering was to make the car more responsive. Kind of like a sho' 'nuff sports car.
It isn't tough to change a car over to power steering, but, it would be pretty expensive. Besides the pump itsel, along with bracket, you would also need a different drag link and ider arm, along with valve body assembly and the cylinder. Not cheap.
