Mill 480x500x180 RIGID

[JAZZCNC]

What do you exactly mean by counterbalance?

Well the name says it all really but how you go about achieving it depends on design.! You can use Gas struts or simple counter weight on rails like you mention.
Last year I nearly bought a large-ish Industrial 5axis VMC overhead gantry machine that had a long heavy Z axis which used simple counter weight on round type linear rails running down the back with a pulleys at the top, just like the Old sash windows.!! Simple an affective with very little setup, aliagnment wasn't critcal.

And maybe move spindle closer to center line of the gantry.

Not understanding you here.? Spindle Nose will determine it's location on Gantry or Plate it fastens on, you won't want the collet nut any higher than the lower edge of the mounting plate.!

But maybe move Z rails on the sides of the gantry at all

Again not understanding.? Why move the rails on the gantry.? Nothing to be gained doing this.!