Gordon,
My practical experience is limited, I'm in the process of upgrading from a wooden framed machine to a steel version, so I would defer to the more experienced builders on this site, but the rail spacing you describe is on the high side of what's common I think. Not that that's a bad thing.

One issue I have with my current design is that the width of the Z axis assembly and the design of the gantry ends restricts the available travel and hence cutting width. I get less than 400mm cutting width from a machine that is over 800mm wide in total. I can raise that to 600mm in the next upgrade with a new gantry design and remoddeling the existing frame (I'm so glad I bolted it together rather than welded it!)

Have a look at pictures of commercial machines and other DIY builds. The fixed rails tend be on the sides of the machine or at baseboard level with the uprights as far apart as possible. This alows the Z axis assembly to travel as far as possible over the width of the bed and beyond. Sorry if I'm preaching to the converted but I actually restricted my cutting area more than I realised while I was concentrating on making the gantry as rigid as possible.

There is a video of the current machine in action at the link below (cutting a wooden gear prototype for a wooden clock. Lots went wrong during this cut but I learned a lot for next time). The current gantry is very much a bodge using many components from the old wood-framed machine and what steel components I could scrounge from the tip (freight costs to Outer WA are horrendous for anything heavy) but it cuts wood reasonably well which is all I want it for.



Kit