Bill is probably right (he usually is) but after breaking the hardwood rails in my P-51 I went with ply and lots of bracing... and no hint of a problem since. I found replacing broken retract rails is not much fun.

My kit was a little short of some of the lumber too (I think), but then I ended up with a bunch of sticks left over that have come in handy for other projects. It worked out well in the end.