Hey Terje,
yes it’s tight there (here you can see what the stock 65 mm mount looks like from the back when it’s disassembled) and since it seems that with this design of the Z assembly on the Onefinity, you would best try to keep the lever short, i.e. the center of the milling motor as close to the rails as possible, I find you considered that very well on your last design. When discussing spindles with higher weight and a bigger distance of the milling motor from the rails in order to clear the stepper, Tom @TMToronto and I indepently from each other came to the idea that the stability of the Z slider would be improved if one would mount two 80 mm mounts on top of the other. You would loose some Z travel though. A bit of loss of travel could be reduced by choosing shorter linear bearings. But I haven’t tried it out yet.
you’re not the only one who is enjoying what you have done here, I am too. Your designs show that for not having the machine yet, you are going very much in the right direction 
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I have also had countless thoughts about this. The Z assembly on the Onefinity is the part I find most worth considering (besides the unappropriate connectors). However, I am not yet sure what I will finally choose. I also still have time as I have to move before I can get the machine up and running.
As for your latest design, I think it would be easier to ensure tighter clearances if it wasn’t bolted together from multiple pieces but milled as one piece from solid (basically). But I’m guessing you don’t have the ability to mill that yet (neither do I at the moment) and that you sound it out with the means one has.
But I’m leaning more towards not keeping the original Z rails assembly though.
And even if I absolutely demand and propagate the possibility to use motors with 43 mm Euro neck on the Onefinity, I don’t plan to use such carbon-brushed universal motors for milling, for me only an induction motor or a synchronous motor come into question.
But for drilling an array of holes into the base of the machine, I would appreciate it very much if I could clamp my drilling machine with reduction gear in it!