Steering Coupling Replacement

I've been trying to replace my steering coupling, and it's been kind of a pain so far. At this point, I just took the horn pad off to try to take the
steering wheel off (in order to remove the steering column), and the 27mm nut holding the wheel down is just completely stuck. Is there another way of doing this? I heard from a few people that I should try removing the steering gear box and remove the coupling that way, but am not sure if that'll work in my car, which is a '79 300SD (W116). Alternatively, any suggestions on getting that nut of without damaging the steering column? Can't just whack it, really...

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