Hello,
FWIW, you should keep the W115 steering box as the W123 was treated to a lower ratio box after a few years of production and the valving modified to make it even lighter, as the 123 is about 300lbs heavier than a 115.
Early W107/114/115 boxes have the highest ratio AND less assistance. IMHO, this gives better feel over the slower and lighter box used in later 123s.

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Nachi11744