Let's say that the plane comes out around 8lbs or so AUW when done using that motor.

That will give you a good 150 watts per pound ( and that motor WILL take well over 1200 watts in bursts ) so your continuous capacity will turn that plane into a balistic flyer... who needs wings? ( This is verified below... )

The web page says the motor is good for up to 7 LiPo cells so you should shoot for that to obtain the stated 1200+ watts that way, or you can use your 6S configuration with a larger higher pitch prop.

FYI: I find that the Monster Power motors will sustain far greater power than the advertising says.

For instance I run the Monster Power .25 which is listed for 3 cells, using 4S packs and I am quite happy with the performance.

However if you increase the voltage you must decrease the prop size to keep the draw down.

Anyway, assuming 6S...

3.7v x 6 = 22.2 volts

22.2 volts x 400kv = 8880 RPM

For an electric that is a low rotation rate, but still workable.

You'll want to prop the motor to get around 7600 RPM or so with the plane on the ground.

The prop will unload in the air to get the final RPM's.

So 1200 watts is about 1.7 HP continuous...

ThrustHP reports that at 7800 RPM a 15x10 APC prop produces around 1.7 HP and an airspeed of around 73 MPH with 9.24lbs of thrust!

That would make the plane fly at a decent speed and thrust.

You can play with the numbers in ThrustHP ( free download ), but you want to keep things around 1.7HP for your calculations.