The only thing I can say, is that I have had exactly the same drama as you. Persistence, sweat and many curse words are normally what gets the new mount on in the end. Seeing as this problem has appeared for me on the W50, W55, W58, A40 and A40D transmissions, which all share the same mount, I would hazard a guess the aftermarket mounts aren't quite made right.
If you have a die grinding burr or a round file, try opening out the holes slightly. Eventually they do go on.
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