I am sure we'd all like to see a picture of your custom adapter. And some details on the source and cost.
I believe what you need to look for is a European 300D without the turbo, and a manual transmission. That is going to have the better-matched flywheel to the 617 engine.
There are a few conversions described on here that you might search for.
Ken300D
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1982 300D at 351K miles
1984 300SD at 217K miles
1987 300D at 370K miles
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