Here's a couple of pics of some other cube interconnects
http://mocoloco.com/archives/000588.php
http://www.lomont.org/Projects/LEDCube/LEDCube2.php
Are you planning a base of some kind? You could just mount each of your 8x8 panels vertically and maybe just glue in a couple of nonconducting supports between them for more strength. That way you only have to run wires down from the columns and the rows are automatically accessible in the base from below.