Yes I understand the reasons you gave for the Y axis saddle to be wider than the Z axis but you aren't machining the rail mounting surfaces to a tenth, so any error in the rail position is amplified by the narrow rail spacing.

unless you intend to use this machine to resurface 500 pound anvils, the weight is of no practical concern, from your drawings it looks like the center of mass is still inside the rails, even with the table fully off to one side, if the bearing blocks are supposidly good to.. what? 20 kilownewtons or 4000 pounds? i think mine are only 30mm long so there is only 20 x 1/8th inch dia balls touching the rail at any given time, so its only good to about 1000 pounds.

Anyhow my experience has been that when you bolt the rail down to the epoxy you're going to get a wave in the rail of about half a thousanth minimum, simply due to the epoxy surface imperfections compressing under the rail.

anyway for the Z axis i would put the rails as far apart as the distance between the spindle is to the column minimum.