I would try the 10 X 6 or 10 X 7 prop, I think the 11 X 6 is a bit much for that engine (it is more of a .46 prop). I would also look at taking the baffle out of the muffler if that is possible, I understand this could give you an extra 1000 RPM, but I do not really know for sure.

Other than that I would look at the options Bruce has suggested, try to make the plane a bit lighter, but even at 6 lbs that .40 should be able to pull it. I have a 6 lb tail dragger trainer that I just flew yesterday with an old OS .30 on it. There was not much power, but with a long take off role it got into the air (with a heck of a head wind - 20 mph).

I think you need the shorter prop and it will get off the ground.

BTW - the .46 should have about 15% more power than the .40 you are running, that may be just what you need to get it off the ground.