[Oldsguy]

I can't say for sure for your '66 but in my '69 the rear end is called a type "O", it looks like a corporate GM 12 bolt to the untrained eye but it isn't. Our big B bodies usually had taller gears intended for use and mileage on the highway, as an example mine is 2.73:1 so you can see there is room for improvement. I have been pondering the prospect of actively seeking a posi third member for the type "O". I was told they were available. Another option is getting a standard corporate rear end with a posi and having the axle adjusted to fit your car if you are into fabricating things yourself or have access. I suspect finding the former would be cheaper but thats just a guess. Good luck.