Guys
I have a 70 Cutlass S with the dreaded 12 bolt cover, ten bolt ring gear rear with 2.56:1 gears. I built a 455 this spring and this just won't do anymore. I see a used Olds 12 bolt posi for sale they say is from a 67 442. It's a posi with 3.08:1 gears. I think the ratio is perfect for me. My question is will it fit? I had been looking for one from a 68-72 A- body. Is a 67 so different? I was hopping to find one of the Corporate 8.5 inch ten bolts. I've seen a few of them but from later years then 72. Anyone know what fits? Do I need to stick with the 68-72 A-body rearend?

 

A 12 bolt Olds rear in 67 is a type O, and the same as your 1970 type O. It will fit, if its in good working order and the price is right go for it.

As far as being dreaded, just keep in mind its an 8.5 rear same as the corporate.

 

The "dreaded" part is the 2.56:1 gear ratio. Both my 70 and the 71 were cursed with that from the factory, but luckily I was able to rectify both of those.