Art,
If I were to build one that size I would probably still build a moving table. Look at the
post of Alvaro's machine. He has a large moving table and it does not look too complicated. I would also do a least 12" of movement. 6" or 12" does not make much of a difference mechanically and it would be a lot easier to get a big piece of material in and out with more clearance.
My second choice would be to make a manually adjustable Z axis lens adjustment. There is no real need to motorize it and it would make the head a lot lighter which will make the rest of the system easier.
At some point I plan on making a bigger machine and I really want to mount the tube on the gantry. This would vastly simplify alignment because you remove one axis from the problem. At the size you are talking about, alignment and divergence start becoming real issues. This would make a heavy Y axis, but all engraving could be done in X axis only. Normal cutting, could still be done in X and Y.