Eric,
I connected the rod to the servo arm with the supplied connectors. (ball link)
It is pretty far out on the servo arm so I can get all that throw, If I remember right it is in the last hole on an aluminum 1 1/4" servo arm.

I don't mean to step on any toes here, but the first thing I would do is get rid of that Hitec servo. In my opinion, Hitec is not a very good servo. I have tried them and had nothing but trouble. They strip out easy they don't center well and they aren't very strong.
I know there are lots of people that like them, but in my opinion, you get what you pay for.

One more thought. I never fly this plane fast. I manage my speed with the throttle and fly to have fun and try wild manuevers. I don't fly it like a pattern plane. If you are flying yours at a faster speed, that may be why I'm not getting flutter and you are.
Just a thought.

Regards, Jim