Yeah, one of my regrets is the design has a mix of metric and imperial units. Sorry about that 
I prefer metric, easy on the math but my mind can only picture imperial. The motor spacer and mount plates aren’t critical as long as they’re thick enough to clear the vee groove but not too far that misaligns the belt too much.
As for tolerance, being no machinist and an amateur at best I didn’t define them, nor sure where to start on how to procure them. Being through-hole mounting of the Shapeoko and my designed parts, they allow for some slop in the system. The biggest factor is the flatness of the plates that mount to the carriage blocks and I wasn’t able to find an specification from HIWIN on the surface mounting outside of the rail height differentiation allowance which is about 0.005" for HG15 series of Z0 light preload. I originally used 1/2" 6061-T6, which has varying gauge tolerance and flatness and surfaced them with my benchtop mini mill to >0.003" flatness across the work pieces. I started to use 3/8" ATP-5 tooling/fixture plate (to cut down on work time and setups, also MIC-6 is an option) which has a gauge tolerance of +/-0.005" (that doesn’t matter as much) and flatness guarantee of <=0.015". Which, well isn’t great for our use case, but due to the short spans of the plates in use (5.5"x9" is my largest stock I start with) hasn’t be worse than 0.005".
The Y-axis is a little bit more complicated because of the dado, which you would want a snug fit (not necessarily press fit). I used gage blocks to get the dado to fit .375" gage block stack and surfaced down the mating plate to fit snug. However if using 3/8" fixture plate, you would just want to use that as your measuring tool. I used stock to leave in Fusion 360 to slowly creep the width of the dado until I got a good fit.
I also see it, It’s still on the base of a Shapeoko platform, it’s still a desktop CNC. I’m not going to try to aim for tenths precision. I mean, I used knock-off HIWIN rails and carriage blocks. They aren’t going to win awards or match HIWIN tolerances and spec’s in my eyes. But they make my CNC much more approachable and desirable results for my hobby use.