Well no, I was thinking of using aluminium, but it will still be quite heavy! Ballscrews will likely be 1610 or 2010, haven't really though about the drilling spindle as yet. Your test on the drilling force (which is pretty huge) probably means a front/rear approach isn't the best as a couple of quick sums indicate that the twisting moment on the gantry is likely to cause significant errors - a "dual rail" gantry as in routercnc's design will obviously not suffer from that, the main deflection would be an upward bowing which shouldn't impact hole position too much.