I did this search for my 66. The 3.25 gear-set is not made anymore, I checked with all the noted gear companies. I found mine in the bone yard. I think it was out of a falcon 8". It was a lucky find because the guy said they are pretty rare. I really like this ration with the four speed.
66 coupe, 289, 4 speed, 3.25 gears
66 coupe, 289, 4 speed, 3.25 gears