I think you should also consider the Advanced Carbide Tools Nine9 10xD Super Power Drills for this application http://www.mctooling.com/index/listings/page1005.htm
9 times out of 10 its reach you need, rather than length of cut, as this length of cut it would have to be a special tool and it would suffer badly with deflection. I suggest you look at making your own shrink-fit extension with a 2xd flute length.