If you can afford the down time on your 280S then "hard" is how much perseverance you have. You'll have to remove the drivetrain and wiring from both cars and install one of them. It will need some mount and driveshaft fabrication including custom exhaust. The wiring may be the biggest challenge because the W124 loom is mostly continuous and very time consuming to remove. Then you'll have spatial differences to deal with and to decide which electrical features to transplant such as the instrument cluster, climate system, etc. based on compatibility.

Someone local to me just transplanted a 3.2L in a W201.

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