There are far more 3.0L 603 engines around than 3.5L, and I would certainly entertain the idea myself -- the 3.0L has plenty of power, and the cost would be about half. I believe a crate long block 3.5 is around $9000 from the dealer, you should be able to find a running 3.0L for $3000 or so. Rebuild cost for the 3.5L will be in the $6000 range for parts, plus about 50 hrs labor, more if the head is in need of work, abint the 3.0L is cheaper since you won't need to do the rods unless bent (although that is the main reason why the 3.0L need rebuilding).

They are physically the same size, so installation is a breeze.

If you were really desperate, you could swap in a 617, too, but that would likely be a major undertaking, probably need a driveshaft custom made.

You could also forgo the diesel altogether a stick in any contemporary V8, too, as long as you swapped the exhaust and computers.

The real Achilles Heel of the W140 chassis is the electronics -- they don't last, and are expensive.

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!