40*42pix = 1680 pixels = 5040 channels
if you plan on 40 strings, and each string on it's seperate ssc, you'd need 3 hubs.
if you zig zagged them, 84 node string run from bottom to top, then again back to the bottom.
you can get away with 2 hubs, and use the matrix option on LSP.
as far as breaking out DMX, if all you need is one universe, then just break it out of the last hub and use the not used channels.
if you need more, then just create an "unused space" in the channel counts on the hub.