When the original Bosch alternator failed on my '84 TD several years ago, I installed an upgrade used Bosch 90A unit - mainly because a friend gave the replacement to me.
I left the original B+ twin-plug harness disconnected but secured with a bracket and sleeve I made from some rubber hose, and fastened to the 90A unit. Then I piggybacked a battery cable to the 8mm B+ stud on the starter, connecting the other end to the 6mm B+ alternator stud.
I used one of those European-style battery cables that has the stamped battery terminal with a 6mm bolt. Then I removed the 6mm bolt and cut off part of the battery terminal, leaving one 6mm lug on the cable to connect to the 6mm alternator stud.
I realize my W123 engine setup is different than yours, but hopefully you'll be able to fit something similar.

Happy Motoring, Mark

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DrDKW