This is the guide I used: http://www.runryder.com/helicopter/t218932p1/

As in the guide I cut my boom at the stock gear box and slid the GWS gear box on after cutting the geared GWS shaft tube off to save even more weight. I only have to move the battery 1/2 inch forward. The GWS gear box is sufficient.

You have to remember in balancing something, the further you get to the end (in this case the tail) the less weight it takes to effect balance. So, you want as little at the end of that tail as possible without compromising performance.