Not sure about the W116, but a W126 300SD swaps directly for a W123 300D.
The W123 MUST be a turbo, the NA 300D will swap to a W123 240D, though.

Best bet is to check for a radiator online and see if the part numbers match....

Jim

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14 E250 BlueTEC black. 45k miles
95 E320 Cabriolet Emerald green 66k miles
94 E320 Cabriolet Emerald green 152k miles
85 300TD 4 spd man, euro bumpers and lights, 15" Pentas dark blue 274k miles