[sphelps]

If the bottom pane is placed on foam and fully supported by the stand the holes can be that close. The glass should however be drilled at a supplier, most glass shops don't have the best equipment. You need the holes to be drilled from both sides to eliminate breakaway chipping which can spread into cracks, especially with holes that close. Also be sure you leave lots of access to those bulkheads, especially the style you plan on using. They are not reverse threaded and come loose easy and often start leaking later down the road requiring tightening or replacing.

It seems obvious you're trying to minimize the space used by the overflow, have you considered an external overflow? Or perhaps moving the return line to the back pane? A lot of people seem to like containing everything within the stand, but running all your plumbing behind the tank and stand keeps things much cleaner and more open withing the stand. Just something to thing about.