Gas to diesel swap for the W108 was fairly common in Europe, dropped both fuel consumption and taxes considerably.

It's quite straighforward -- the engine will fit directly on the mounts, and I believe even the driveshaft is close.

Not free, though....

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!