To say it better then, you will need to design a table with rails and gantry and then add separate bolt on working area which has to be higher and hide the rails. Otherwise you will be facing an impossible problem of aligning the rails.

Its up to you of course, but maybe for such a machinea separate working table is better idea, and all else modular design using RP, like heavy duty plasma tables.
Cause at the end of the day this is a production machine. Tomorrow you could decide you need 5 meters longer or sell to sb who need 1 meter longer, etc...

Adding then ATHC and plasma torch is all needed to convert it to plasma , and so on. means easier to sell if needed an upgrade.

Of course your priorities could be different.

Using ball srew and rotating ball nut on such a machine could make it very high value machine that can precisely cope with big stuff. Combine that with strong gantry and you will have a real beast