Any MB engine will drop right in and sit on the mounts correctly. However, the problem will be the wiring and computer systems.

The M104 uses a different computer and EFI system, so that will require $$$$$ and time/effort, along with custom wiring unless it was used in the W126 (I don't know if it was).

V8s are easier, as they were factory. You would then need a set of front springs, maybe, unless the wieght difference is small.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!