I ran into a similar problem when transplanting a W108 280S engine (1968) into my W108 250S (1967). The 250S has an oil cooler that is NOT part of the radiator. It's a heat exchanger that resides near the left (driver side) engine mount arm. Since the 250S came without an engine, I was in search for an oil cooler that would work with the 250S radiator. What I eventually did was find a junked 250S and get all of the plumbing for the oil cooler and add it to the 280S. Worked fine.

I _think_ I couldn't use the radiator from the 280S because it was 2 inches to wide to fit in the opening of the 250S, so maybe we are talking about exactly the same problem. You might want to look in EPC and see how the oil was cooled on a 250S. Might provide some ideas if nothing else.