I'd like to modify the suggestions a little so far. Removing material from the bottom of the tee nut is the normal way to address this, because if you recut the two rabbet cuts, you do need to pay close attention to the parallelism and accuracy of matching the two cuts to evenly distribute the pressure from tightening the post to the top of the compound. You could conceivably even crack the compound in an extreme case. Removing material from the bottom is far less critical, since no contact is made against this surface when the toolpost is mounted. All force is applied upward, against the two shoulders. I have seen other guys even remove enough material to fit the slot with an angle grinder, test fitting as they went. Wasn't pretty, but it worked. The case being, the bottom does not need to be absolutely parallel with the top, or shoulders as long as there is clearance in the slot. Ideally, there is not excessive clearance there, as thinning the tee nut weakens it. So if it is possible to maintain a reasonable parallelism, then only a minimum needs to be removed and the tee nut stays as thick as possible.
