I stand corrected. Got out my big calipers and my original '69 measures 6-1/2" across the case and 7" on the outside of the band. I think this is known as the "Second Generation" booster.
The later one measures 7" across the case and 7-1/2" across the outside of the band. This is the "Third generation" booster.
Chrysler had a dealer install booster kit p/n 3420972 for '67-'70 Dart and Valiant and '67-'69 Barracuda. It was the larger 3rd generation (7-1/2" ) booster.
Jim