I see what you mean about the plate. You could easily offset the master cylinder to the passenger side and use the upper portion of the W115 pedal as is. The W115 pedal will miss the brake switch but a small tab would take care of that.

I do think you'll need to splice the lower portion of the W123 pedal on since the W115 will end up high and too close to the clutch pedal. If you look at the image "Arms Compared2" it seems to me you could cut the W123 pedal below the indented area where the master cylinder attaches and splice it directly to the upper portion of the W115 part. You should end up with near perfect geometry for your car and a straight section to relocate the pushrod.

I still think the issue of providing a stop needs to be addressed or the pedal will sit too high and the brake switch will take all the punishment. It isn't a huge problem but it will be a factor in determining the thickness of the plate.

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1969 220D 5 Speed (OM616)
1983 240D 4 Speed
1985 300D Auto 376K
1985 300D Auto 275K