Putting a 420 or 560 motor in the 380 isnt that hard. As far as exhaust, the only thing that would change would be the headers, which I believe to be the same. You can reuse the old ones I believe. Euro versions have two pipes instead of the USA ones that collaberate at the back of the engine and feed to the pass. side. As far as weight goes, the 420 motor is probably lighter than the 3.8, being all aluminum alloy, block and heads. But if you were to put a 4.2 or the 5.6, you would probably want a doner car. Since there are alot of little things here and there that you would need. I personally would pick up a 560SEL, instead of sinking 6000 into such an old car. But for some, the car does have sentimental value, and I guess you cant put a price on that. just my two cents.

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93 300D 2.5 Turbo, Black/Palomino 273K
09 E350 Black/Black 41K