As far as I can see you are not doing anything wrong, & I'd like to be helpful, but I don't have any experience with this particular part. I'd say try to splice it & see what happens I can't even envision that cable application so I don't even know how much room there is to make a splice & what kind of splicing options might work.
This cable doesn't actually make the shift at the transmission, does it? If it did, I'd say be cautious about changing the length of the cable as I think the potential to harm the transmission might exist.
It's kind of hit & miss on these forums, so something else you might try is to post in a dfifferent Gen1 subforum & you might get more action.
But good luck with this.