[bodywerks]

Chances are pretty good that you will come out a little noseheavy. I would just mount the servo in the tail. Aeroworks should have put an optional rudder servo slot back there - they do on all their other planes
If you are bent on going with the extra complication of a pull-pull setup, get the SWB 2-1/4" full arm(4.5 total length) and put the ball link on the 3rd hole in. I also recommend putting the ball link on the bottom of the servo arm so that the load is placed more in-line with the shaft bearing. MOunting it on to induces a tortional load that can cause premature wear on the servo.
The Aeroworks control horns also have three different holes that you can mount the ball link to, so you have options. In fact, I am only using 1.5" single arms on my 42% Aeroworks, so I don't see how you would need more than a 3" arm.
Measure the distance from the hingeline to the outer hole on the control horn. It shouldn't be more than 1.5", if that.