No change shall be made to the steering gear, linkage, or related parts that would alter the manufacturer's intended geometry, nor shall any addition be made that would unsafely affect the operation or stability of a school bus. On school buses constructed after January 1, 1950, the outer rim of the steering wheel shall be at least 3 in. from the instrument panel, windshield frame, and other obstructive surface or mechanical device except the turn signal lever and a gearshift mounted on the steering post.