The B9 upper arms are functionally the same length as the rest of the production cars in 69 and 70 - that is the same distance from the ball joint pivot point to the inner shaft pivot point.
There has been some speculation that the B9 arm is thicker metal or that the shock pivot point is moved farther outboard.
Anyway, the B9 Mustangs had the inboard lower arm pivot moved out 1/2" on each side and the new shock tower (which was welded in as an whole frame/apron/shock tower assembly at the Dearborn plant) must have moved the upper arm out 1/2" to match. So the front track is 1" wider. The front gravel shield had to be modified at the wheelwell to clear the tires on the B9's and the front fender lips had a tighter radius.
