That's because it wasn't an option for 6 cylinder manual transmission in 1966 In the 65 manual you will see the 225 listed under the 4 speed options and you will also see the 309 1st gear where in 1966 you will see the 2.66 first gear listed .The small clutch they used couldn't launch the car right with that high 1 st gear and would wear out in short order which is why they discontinued the 4 speed option for 6 cylinder for 1966.
