What prop are you using now? I agree with experimenting with props first, but I think 10 x 4 is too small to be efficient on that engine--the 11 x 4 suggestion sounds more like it to me. I've seen LT-40s at our club fly well with the 40 LA which is about the same power as the FP.

I found my 40 FP did very well on an 11 x 5 on a trainer and 10 x 5 on a much smaller Dynaflite Mustang. The LT-40 is a big, light trainer and my first try would be to go down a notch in pitch and keep the 11 inch diameter.

If I were buying an engine for it from the start it would be a TT 36 Pro, or something like it. It's light (same as the OS 40 FP) but plenty powerful even on 11 inch props, and Radical RC sells them for about 65 bucks--I think it might a better choice than the 46 LA for a similar price. But try different props first--good learning experience and might solve your problem. Jim