Rod, the GP Cub has shear webbing in the plans and it is 1/16 balsa. You just need to make sure the grain is going in the right direction.
I cut my own spars and I have used pine but found it just a bit brittle. That and finding a clear board without knots is often hard. My local high end wood store doesn't stock Bass wood so I have tried several different woods for spars. I even have some cut from OAK, strong but heavy!! I found a stick in the scrap bin and was in need of a couple of 3/8 X 36 so what the heck!! I have found the poplar a pretty good spar. It's a light weight hard wood but it is strong. I can find it in pieces of clear boards with almost no knots. It cuts really well!! Last plane I built called for 1/8 X 1/4 TE stock. I ws able to cut a bunch of it in 10 foot lengths. It even dazzled the guys in my wood working club that I was able to get it that thin and long without breaking. Well, several didn't break. I'm not 100% one with perfection but I have several sticks of it in my shop in 10 foot lengths. We do have two of the best table saws I have ever used and I use new blades!!
Gene