OK...part number for the 617.910/912 manual flywheel is:

617 030 00 05

Difference in length is 74.0mm vs. 90.0mm for the 300D, it's heavier but doesn't say by how much.
The SI is pretty explicit about balancing and not mixing them up and installing the wrong one, but in
this case I don't think it matters too much, since there is no "right" flywheel for the turbo anyhow.
The differences likely have everything to do with engine tuning, balance should still be OK.

MB list on this part has gone waaaay up to $515.00.

Quote:

The modified (for the manual conversion) driveshaft was made up from a remanuactured driveshaft (which to the point that the conversion was started, did not have any vibration) and the shortened front stub from a donor 240D. The work was done by a reputable shop and it was balanced after the fabrication was completed.

Hmmmm, totally grasping at straws here, but IIRC, the n/a and turbo driveshafts are not the same diameter.
If the "stub" end is from a 240, and the rear from a turbo, that could cause a vibration, I'd think moreso under
load. I could *definately* be mistaken on that though, it's been a while since I've looked at one, I just seem to
remember the turbo driveshafts being fatter. Maybe it's only the rear section (or all in my head). The
part numbers between say a 300D auto and 300D-T auto for both sections are not the same (I'm figuring those should be the same length, maybe they aren't), but I'm not sure how else they differ if not in diameter/weight, and aside from the different flex discs they ran between turbos and n/a cars.