If, in fact, the ratio of the intended "W126" center section is 3.06, then it has a ring gear diameter of 210mm, and is known as a "large" center section. However, if the ratio of the chosen unit is 3.07 it will have a ring gear of 185mm diameter, and will be known as a "small" center section.
If it is a "small" (185mm) center section, it will interchange directly with the one in the W123 chassis.
If it is a "large" (210mm) center section, shorter half-shafts (axles) will be required.
The circlips are accessed by removing the cover of the center section.