I can't offer an opinion on the FP(They are from before my time in RC), but have flown an LT40 with a 46LA and with a 46FX. Now, keep in mind that I fly from a relatively small field surrounded by trees (Actually it's "TREES!!!!"). After take off, you have to start an almost immediate climbing turn to clear the trees at the far end of the runway and continue climbing on the down wind leg of the circuit to get over the trees at the far side of the field. When I flew with the 46LA, take off and climb out was sometimes a little more "interesting" than I was really comfortable with. I replaced the LA with a 46FX and the difference was quite impressive... there was a lot more margin for error and things were a whole lot less "exciting" on take off.

I will admit that some(all) of the climbing ability(or relative lack thereof) with the LA could well have been due to my lack of skill at the time(There some who tell me that I still have that problem!). Also, I still fly the LA (but on a smaller airplane) and it has been a good performer, so I'm not trying to knock the LA.

What I would suggest, is talk to some of the more experienced guys at the field you'll be flying at and ask their opinions of the FP40/LT40 match. You may find that it will work, or you may find that they'll suggest a larger engine.

Good luck!