Lacking people giving evidence to the contrary, I tend to agree with you. I finished slotting the holes with a file today. Not the best tool for the job, but got it done. My file was just a tad big, and I could only use the last 4 inches of the taper. I made some plugs to fill the front part of the holes and just put them in place with JB weld. Once in , they will be captured in place, so I'm not worried about the JB weld failing. I wouldn't be surprised to see it intact years later though either.I would think you will need to fabricate something to keep it from sliding forwards. And not just something to suggest it does not slide forward, something that PREVENTS it from sliding forward. The forces involved here are quite high, when the winch is applying a given force to pull and equal yet opposite force is pulling the winch forward.
Maybe a square bar of steel between the front lip and the base of the winch that is the right thickness.
I'll do a more complete write up of this in another thread. I will probably finish things up tomorrow and post up some finished pics here.