Darren, Fir is a good choice, I have used fir for my airplane. The complete fuse without plywood weighs 22 pounds, if I had built with Spruce it would have weighed about 19 to 20 pounds. Not much difference if you ask me. Truth is, the frame of the airplane only makes up a small part of the whole airplane, if weight is a worry for you (should be) try to save weight somewhere else like in your seats or wheels and brakes.
I used Fir because I had a great (read that as free) source for it here otherwise I would have used Spruce. I do agree with Steve Jacobs though, hoop pine is available to you "down under" and is a great substitute for either and probably a lot less expensive. Good luck! Fred Johnson Reno, NV
