MECHTECH - please don't give up on this one, I think it can be worked out just fine. The thing here is they brought out a model of a vehicle that is pretty exotic... I don't know if there is an example that survived the war. If there is you can try to look for pictures. But if none of them babies survived - then you're basically there where the Trumpeter designers were trying to design something reasonable.
Now one might feel the urge to look down on them boys (and girls?) from Trumpeter because their research isn't as good as it should be - but that is the payoff for getting SOMETHING that otherwise would have to be scratchbuild and/or kitbashed. Now you have something that's almost there and you just need to work out minor details. I always look at something like that as an oppportunity to develop skills...
So I agree that this would look good along the Karl, and you got a good price for it - let's make it work then. Did you look up google for "Munitionsschlepper Karl Geraet"? There are some photos there, but yeah, they need interpretation...
Looking at all those pics also made me think how hydraulics made the life easier for the crane designers...
OK, so I took another half an hour to look at various crane pictures and diagrams and I came to a conclusion, that most sheaves (pulleys) are used for the luff rope (the one to move the boom up and down). I have marked the luff rope green and the hoist rope red. Here's the diagram I came up with:
Now you definitely need two pieces of rope for this - luckily making two out of one (cut!) is easier than makin one from two pieces... Also, from what I've seen I wouldn't take the kit supplied twine, but something a lot finer - maybe fine fishing line or something like this.
I sure do hope this helps you - I really invested in this one...
Good luck with your model and have a nice day
Paweł