The GP P-38 builds a bit heavy for the wing area, you might try flying the plane before you go and add the cowls.

While we are on the subject, here are 2 lesser known manufacturers of P-38's

http://www.glhobbies.com/MAIN.htm
http://www.rcairplane.net/index6.html

Haven't done any research on the build or flying yet, any experience with them will be appreciated.

BTW I am building a Wing P-38, only comment I have on it so far is that it is a sport scale model, mainly built from flat balsa sheeting stategically carved, so perhaps not the one to get for any serious scale project.