I have two 12 bolts, one is the '67 width and one is the '70. If I recall the 67 is 1.5" shorter on each side from the flange to the lower control arm mount, so a 3" total. I could verify tomorrow if you need it. Not much but enough where when I swapped them around the 10" wide slicks I ran on the 67 rear wouldn't fit on the 70 rear without the quarter panel wheel well lips hitting the tires
1.5" sounds right to me, clearly Brian has a better handle on the mods. Just wanted you to be aware...and it's true that rear is worth $ to some chevelle guy,
The difference in width is 5/8" on each side. The Chevy rears were narrower from 64-67,then went to the wider rear is 68. Buick,Olds,Pontiac went wider in 66. Some early 66'ms may have the narrower rear,but for the most part BOP was narrow for 64-65,then wider for 66-72. When converting from a BOP 8.2" 10-bolt,to a 71-72 8.5" 10-bolt,you will need to shorten your driveshaft 3/4"-1". Why? The 8.5" 10-bolt has a longer pinion snout,so it needs less shaft to reach it. You will need to do similar driveshaft mods when swapping from any of the other GM rears- 8.2" Chevy,12-bolt Chevy,12-bolt O-axle.
Wow my memory was off a bit I went out and measured both rear ends and came up with close to what Brian says. I thought it was an 1.5" on each side but it's about 1.5" total. I had 3/4" difference on each side. Close enough!
fwiw late 67 chevelle 12 bolts are same length as 68-72 12 bolts,early 67 and 64-66 are the same length.
My old notes show: '64 Olds F-85 8.2" 10 bolt= 60" drum-drum '66 Chevelle 12 bolt = 60" drum-drum '66 GS 8.2" 10 bolt= 61" drum-drum.
I installed a wide Skylark 1971-72 8.5" 10 bolt into my 67 GS and then I had a circle track guru widen 4 original 15" Buick ralley wheels by adding a one inch spacer behind the center flanges, hidden from view. He cut and welded them on a special lathe. I figured the 1" spacers would bring the tires away from the fenders and closer to the frame where there was extra room to start with. It worked! Now my 7" Buick GS ralleys are 8" ralleys and I have perfectly centered 275-60/15 MT Street radials under my car. A "guesswork" turned perfect.
