I've changed out a box on a w123. It took a couple of hours and that is doing it by myself. I used a regular floor jack to get it up there.
But because there is stuff in the way to get to it's final placement, you can't use a floor jack to get it the rest of the way. You might be able to use a bottle jack though, but I didn't do that. I bolted the steering rod to the coupling which help in holding it up and the last part I scooted the jack aside and muscle it in while putting a mounting bolt in.
Regarding the spline. It's not that hard. There are hash marks that tell you where to place it. It's basically set at 90 degrees from the frame of the car.
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