After some thinking, it may make more sense to modify the Tormach casting rather than the Z slide on the X3. I took some measurements and did some sketching last night and it looks like I could slot out the holes on the Tormach head to line up with the X3 Z slide holes. There’s plenty of material on the head mounting flanges to do that. The Tormach holes are not directly centered (left-right) in the flanges, but instead offset towards the outside edges. The new holes need to move inboard 8mm on each side. Looks like there’s plenty of room to do that, and still allow the backing washers for the bolts to sit flat on the flanges.
I could make some plugs and either press fit or slip fit + red Loctite them into the Tormach holes. Once in place then I could machine the new mounting holes. Maybe this is overkill, but on the whole I think it’s less effort than my other approach.
I also like this plan a little better because it means I can do all of the work on my own machine. My friend’s manual Rockwell mill that I was planning to use won’t be available for a while. Another bonus is that I wouldn’t be making any permanent changes to the X3 and could go back to the original head easily.