Your going to have a weight issue with the .61. To offset the aft CG with the .61 mounted in the back your going to need allot of weight in the nose. Now your wing loading is up so high that the plane will fly like Sh%@#t. I have a YS .45 on mine and am going to put it on a diet. I'm going to shift the radio, servos and battery pack as far forward as I can, so I won't need ballast.
I just haven't had the time to work on it, but it's on my list of things to do.
Regards,
Chris