The frame does appear to be different between the early and late setup, but there is a significant difference in the brake pedal itself as well. The early brake pedal curves around to the right of the column while the later brake pedal is to the left of the column.
Had a friend that bought a '74 Duster with a 318 and a 4 speed swap done on it. He was having problems with the brake pedal hanging up and we discovered that the 4 speed swap was done with an early brake pedal and it was dragging on the steering column. So I would postulate that an early brake pedal should not be used in a later car.
I think when the A-Body was widened in '67, the top of the column moved out while the steering box stayed in the same place or at least didn't move as much. This caused the column to be at an angle and put it up against the brake pedal, so they moved the arm of the pedal over to the other side of the column resulting in two different brake pedals.