Wow, everyone's turning into PTW whores (no offense, but you are :P )
@OP:
First off, I love KA but that gearbox won't do; it's made of zinc alloy. I don't care how reinforced it is or how big the bushings are, zinc alloy is shitty. The weak point on the v2 box isn't the bushings anyway, it's the front of the box (in front of the cylinder).
If I had to do this, I'd go to a complete Guarder mechbox like this:
http://www.intrudershop.com/show_pro...idproduct=1064
More expensive, but made of steel and designed to work with Guarder's higher end springs. Add one of them (the Guarder SP160 will allegedly take you to 500-520fps) and you pretty much have your mechbox.
One thing I'd add since you're using a MOSFET and [hopefully] a larger battery is a sector clip, to insure proper feeding (specially if you plan on running semi a lot). And switch out all your connectors for Deans.
I'd maybe throw in the extra $3.50 and swap out the stock pinion gear on th emotor for the heavy duty hardened one, too.
And +1 LUNIT's comment about 6.03 over 6.01.
EDIT: btw, I've never built a 500+ FPS mechbox; I'm just saying if I had to, based on my experience this is how I'd attempt it. It may blow up in your face for all I know, mmmmkay?