Dear Chris:
I did this job on my MB 300 SD - 1982 a not long ago and I had the same situation that you are having now.
The only and easy way that I found to remove that superior allen bolt was using a joint between 10 mm allen wrench and extension bar with a good breaker bar. I had also a hard time to get that bolt loose because there are not room to work but I got it done using the joint as I said above.
Hope it work for you.

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Nyck - TX - USA