Hi Steve,
Joe won't be out of bed yet

There's a fair amount of stuff on gantry design on the forum. Putting rails top and bottom has the advantage of pulling the Z axis closer to the gantry and reducing the overhang. It also gets the rails as far apart as possible for the chosen height of the gantry but makes the construction of the Z back-plate more complex. Alignment of the rails is not 'automatic' in any way and must be done with care.

The option of mounting rails directly on the front of the gantry pushes the Z axis out but can be easier to align, especially if you're using epoxy leveling or a machine-ground plate to ensure a perfectly plane surface. It also makes the Z axis back-pale into a single flat piece. It all comes down to what tools and measuring instruments you have access to.

As long as the first machine you build is adequate to cut out the pieces for a second, better version then you will be on an upward evolutionary slope.

Kit