Another option is to use your 300SD engine along with its 722.303 transmission.

You would need to convert the speedometer drive to a mechanical gear using the tailplate from a 722.315 (300D turbo) transmission, and then get all the W123 linkages from a 300D turbo, and bring over your 300SD vacuum control valve etc.

But I see no reason why the bellcrank from a W116 would not work in a W123- it doesn't stick out into the chassis too far IIRC. Using this 722.1xx transmission behind a turbo engine might not be a good idea.

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