The 40 LA will be a little weak for the Somethin Extra. If you are a beginner, this plane will be a little much for you. On the needle valve, unscrew it and remove it, leave the housing body on place. You should be able to now fit the engine in place. Visualize now where the valve will stick out the side and drill a hole large enough for it to fit through easily. You will have to seal the exposed wood (epoxy, white paint or both). It looks much better than cutting a huge U shaped gouge out of your new plane. As with all ARF's, a must is to re-iron and seal all the covering edges and overlaps. Use finishing epoxy in the engine compartment, anywhere fuel and oil will touch. The landing gear mounting area is weak and the gear will rip off in a not so nice landing. Added angle bracing and epoxy on the inside will help.
Have fun with it, I hope you have a computer radio, if you do dial in at least 60% expo for the ailerons and elevator. Go easy on the control throws also. Hope this helps.

Jeff
http://www.angelfire.com/wa/JeffMDuffield/rcplanes.html