The 73+ disk brakes added width compared to the 67-72 drums/disks, but that was a difference in the spindles and hubs, the upper and lower control arms were the same length and the K’s interchange. The 64-66 cars were slightly narrower in the k frame and frame rails, they also used the same length control arms.
As for narrowing it 4”, good luck! You can’t shorten the arms that much without causing some real grief when it comes to suspension geometry. Your only other option would be to move the suspension mounting points in, but, that would basically mean moving the frame rails in. Since there’s already barely any room for the engine, exhaust and steering box I don’t see how you’d do that either. And that would also change your suspension geometry.
The real question is, why do you need/want to narrow it that much?
