The post is confusing on several issues. Photos always help. As mentioned, 1966 cars did not have the front ball & trunnion U-joint (originally Detroit brand). But, you are asking about the rear U-joint, which I think would be the same PN as 1963-65 cars and 67+. What probably matters is the rear-end. Is it 7.25"?
For those needing a 64-65 B&T front U-joint, I posted a list of PN's several times, similar to post #5. I don't have at hand, but Search works. You can occasionally score some on ebay, if patient. I am set for life on those.
