It looks like the flywheel balancing from the 240 and shortening of the drive shaft will be the biggest issues. Of course the engine/ auto trans and diff from the 300d would be a direct bolt in. The 4 speed would give it more zip but it would be more money and hassle to do it . I wonder if the driveability difference would make it worthwhile?

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92 300e 205 k miles decent -wife's ride
91 300 TE many upgrades and very clean-Junk yard rescue ( son's "hot rod")
68 250 s Low rust great shape
91 300e 236k ( rear ended and sheared metal subframe mounts /sway bar )
85 300TD 310k

Below have moved on
77 300d 36k miles one elderly owner! Now in Ca fully restored
79 300 sd 49k miles( time capsule clean )sold and still in ohio
66 230s new at factory "das boot" gone.
93 190e 2.6 crunched-hard no injuries
69 220d rusted away