A 9X7, 10X6, 11X4 or 11X5 would work well with the LA. The 12" prop you have won't cause damage to the engine if the plane stays on the ground. If you put a 12" prop on that .40 in a .40 size plane, you might have enough power to take off, but turns are going to require a lot of altitude. The LA series are concidered generic type engines. They won't have enough power to take the plane through a turn without losing altitude or stalling. The prop you have is better suited for a high end .46, a .55 or .61 engine. On your LT-40 / .40LA combination I would recommend a 10X6 or 11X4.