Hello Lorenz

That is funny - but understandable.
I think you have solved it.

The original adaptor plate appears to have been cast. But who knows?
Perhaps it was two steel plates welded together to get height and angle.

I guess the ideal centre-line would be between the crankshaft and
the centre of the drive pulley.

Here is another photo to assist:-

[Linked Image]


Hope this helps.
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Jack