The manual that you need is for Gl 76/27 gearbox.I have it but it is not readily available at the moment. I can email it before too long.
Do you know for sure that the linkage is correct? I would get under there and be sure it is in gear all the way. A few other boxes have parts that are the same as these even the lower torque models. I forget the GL designation Might be 76/18. Either way those correlate to 717.411 through 413. The 717.43x. also comes on the G Wagon. Those are a harder gearbox to get, as you already know. Surprisingly, your box and the one from the 201 models share quite a few components. I wish I could remember what, but I think that the gears were the same. Give it a look. The 717.400 is a different type of gearbox.
You should be able to tell if that later model transmission mentioned would take the tail pice from another by carefully checking the EPC. The EPC is also critical in determining what parts interchange between the . Mercedes did that for a reason as it states in the manual. There is an excerpt in the manual about modifications that they made for improvements. The reasoning for some were "standardization". Edit, I just looked, and the regular and very common 4speed also may share some parts with the 717.430. If I remember correctly, the gears were the same while the shafts were not, but it has been a long time since I looked. If I had the time, I would. Just out of curiosity.
I will get the manual as soon as I fire up this old computer. 4X4abc.com has them for sale I think. He is also on BW with that handle minus the .com. I'll give the manual for free, but might be a minute.

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Chris
84 280sl
82 300d euro