Since these frames are designed to flex when traveling over uneven roads, mounting the cab solidly to the frame, front and back, would cause deformation of the cab. The shackles allow the cab to articulate when the frame flexes. If you had severe wear of the bushings, you would have problems with pedal alignment, and weird noises, but I don't think you will get much improvement in the ride. BTW I have replaced three sets of these rubber bushings on 3 AD trucks and had a hard time getting them out. If I didn't need to clean and paint the shackles, I would have left the old bushings in place.
