Rear end question
 

I spoke with a guy who has a 70 cutlass with a 12 bolt rear. He said he wants to use it for his 66. He said that if it does not fit his 66, he would consider selling it to me for my 68. Does anyone know if that rear will fit the 66?

The 70 rearend is a 1" wider if remember correctly. Olds does not make a 12 bolt they make a rearend with a 12 bolt cover that really is a considered a 10 bolt. My advise is to run from it. Parts are extremely expensive for it. I had one when I got my first chevelle that claimed it had a 12 bolt under the car. Find a chevy 12 bolt for your car.

If it has 12 bolts and a smooth cover, it's not worth using.

If it has 12 bolts and an upside-down "V" in the cover to direct oil from the ring gear to the axle bearings, then it's a Chevy and definitely a good one.

Thanks for the info. It is a 12 bolt with smooth cover. I will pass.

To answer the OP question...
Yes that '70 BOP rearend will fit your friend's '66 Chevelle. Installation will require a set of '67 Chevelle coil springs...will have a pigtail on both ends unlike the bolt-in '66 coils which have only one (upper).
The '70 rear is about 5/8" longer on each axle tube but this additional width can be overcome with an increase of wheel offset.