There's always the BMW GS637DZ option.

https://dieselpumpuk.com/products/om617-to-bmw-6-speed-37dz-adapter-kit

This is the 6 speed transmission I have in my W123 .

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Old Blue - '83 240D 6 speed manual trans, OM648
Green Machine - 2001 Europa G500
'87 300SDL Bought on Peachparts
'02 E320 4matic wagon

Patchouli Wagon - 2004 E500 - SOLD
333k mile 97 BMW 750il V12. - SOLD
The Californian - 85 300TD - SOLD
Daily Super Sedan - 03 Audi S8 - SOLD
Family Truckster - 2012 VW Touareg TDI - SOLD