Have a look at a thread made by AdrianM building a GP Extra 300 .60 (Mods for a GP Extra 300s .60 size for 3d). He describes how he enlarged the rudder and elevator. Have in mind that there are many simularities between the 40 and 60 size. Myself followed many of Adrians modifications. One about the rudder is to extend the bottom of rudder with 1 inch (backwards) and then taper the trailing edge of rudder up to stock location at top. This to minimise any roll coupling when flying eg knife edge. Considering you are building the 40 size, I would do the very same but extend with 3/4 of an inch tapered up to stock location (upper tip) to counter the slightly smaller plane compered to the .60 bird.
Good luck,

Christian Sweden