The 68-up (until they started using the 8.5) rear end is completely different--it was Buick specific. The 64-67 rear end was a Pontiac design that Buick bought from Pontiac all those years. They are both considered 8.2, but there are alot of significant internal differences. The biggest differences are the big pinion bearing is alot bigger in the 68-up rear, it has 7/16" ring gear bolts instead of 3/8", and the left and right carrier bearings are different sizes in the 68-up rear end.
If you look at a set of upper control arms from a '64-'67 Skylark, you will notice they have "Pont" stamped in the top of them.