I am doing a similar modification on my axe but I am not using freestyle-hobby kit.
The freestyle hobby kit is simple and cheaper but it is designed around the stock main gear.
I wanted to change the main gear so I am not stuck with the axe pitch which is usefull if converting to brushless.
So I use parts for the blade CP conversion and t-rex main gear do a little bit of modification on it. the stock boom will work but 397T belt is needed because axe CP boom is longer than blade CP. I am almost done with it, I am just waiting for some bearing that I am going to use on the guide pulley.