Use the .61. You will be a lot happier. I built on last winter and spring. I did a lot of conversions that made it a little on the heavy side. I turned it into a tail dragger and made it with struts and bolt down wings. Hate rubber bands. Anyway I put a Magnum .52 4-stroke on mine and it's ok, but I wish I would have put a .70 on it. Mine flies fine but it is a bit slow. This plane is meant to fly slow and stable but the .40 will be a dog. I know someone that has one and his is a real dog.

Gibbs