Thanks for measuring. FYI I did some quick CAD measurements and you will see the true spacing should be actually 57.15mm-53.0mm=4.15mm. See image
You are correct. I had to use smaller spacer washers inside the V wheel assemblies after using 3/16" aluminum tee, due to the fact that the center of the V rails was no longer in the center of the V wheels. 0.032" fiber washers worked like a charm.
Those are some gigantic V-Wheels in that drawing.
It is to scale the outer yellow spacer washer(1mmx8mm), blue bearing (5mmx18mm), there is a center washer(1mmx8mm). Also got the cad image from openbuilds part library. The images are not in scale with each other but dimensions listed are correct.
“Everything is fixable with violence and vazeline”.
Right? 
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If all you have is a hammer, then everything starts to look like a nail…
Of course, if you have a big enough hammer, that’s not necessarily a bad thing…
Hope that’s a metric mallet and not imperial rubbish!
Please, ignore my two previous posts. I did miscalculate the rail-to-rail distance. What I suggested was the same as @jzhvymetal earlier which is narrower by 4.75mm (3/16").
I will delete the above when possible (it seems there is some limitation in the forum).
However… the target 44.75mm width can be achieved by flipping the linear rails (still on a 40x40 V-Slot) so that the rails stick out (instead of hugging the body of the 40x40).
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However… the target 44.75mm width can be achieved by flipping the
linear rails (still on a 40x40 V-Slot) so that the rails stick out
(instead of hugging the body of the 40x40).
I thought about doing that but I’m not sure without supporting the V on the rail it will not deform. I’m going to try removing the 1mm spacer washer and use 3mm spacers to space out the wheels.
Either way should work.
In the flipped rail solution, it is not support I would worry about. After all, there’s a bolt every 50mm (40 bolts in total for the 500m version, 80 for the 1000mm !! ). It is something else (see below).
The way I see it, both solutions address the X axis rigidity issue, but each has a pro and a con.
flipped version (44.75mm)
pro: original spacers, less stress/potential torquing on the V wheel axes
con: free/guide-less alignment
normal version (40mm)
pro: easy guided alignment
con: extra spacers, more stress/potential torquing on the V wheel axes
A small reinforcement you could try with the normal & spacer solution (or the flipped or the original for that matter) would be to replace the top four M5x25 screws that hold the four top V wheels with two M5x80.
This way you update from four stubs to two real axes, which eliminates any worries of micro-bending of the top V wheels.
It can be tempting to do the same for the bottom four V wheels, but this is tricky due to the eccentric nuts. You would have to cut off the head of another pair of M5x80 and add another four extra lock nuts to hold the V wheels’ internal side. Then adjusting the four eccentric nuts independently will be the challenging (but not impossible) part.
Is there a chance that the M5x80 screws would rub on the belt?
Edit: I haven’t looked at it to know… It was just a thought
No, they clear the belt by a lot. They already do.
If you take a close look, the current M5x25’s are 15mm apart… so close, like they want to kiss
An M5x80 there would provide a nice solid bridge.
Cool. Nice idea.
UPDATE:
While all the attention was given to align the V wheels, a very significant detail slipped through. In case we use the 40x40 V-Slot (or any other 40x40 extrusion), the stock holes on the Y plates will not align! They are spaced at 24.15mm and not at 20mm that the 40x40 expects. Also drilling two new holes on each Y plates is not an option as the new ones would slightly overlap with the old ones.
Therefore, 40x40 could be a good choice as long as one is willing to build their own Y plates. Hey, does anyone here have a machine that can mill aluminum? 
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Rather than modify M5x80 (or longer) screws, better to use all-thread rods which most hardware stores will have.
Discussion of this here: Shapeoko CNC Router, Rigid, Accurate, Reliable, and Affordable
Answering to myself 
…actually, since the Y plate has two holes and two slots for mounting the MakerSlide, the mod for either of the 40x40 solutions can be done by drilling. Keeping the two (round holes) as guides for squaring and expanding the slots by 2.65mm (towards the round holes) should suffice.
This could be done on a spare set of Y plates (total of $37) if we want to save the originals.
Here is the cad with the dimensions. The holes and slots in question can be seen bellow (top right corner).
@WillAdams good point.
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In order to keep this thread on topic, I started a new one on the Y-plate mod, since this is a prerequisite for the 40x40-extrusion-based solutions discussed here.
You will also find
- mounts for a Y-max limit switch
- and extra holes for solid V-wheels as an upgrade path to Y-axis 40x40 V-Slot (i.e. Y-axis stiffening)
I named it “universal” as the original idea was to make it compatible with MakerSlide, V-Slot and C-Beam (the latter is not included yet in v1).
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Jeff, I absolutely love the work space you have designed for your X-Carve. Having the side table for the computer is a great idea. That is why I love this site and the graciousness of the people that share ideas. Thanks