On most of my scratch built 60 size warbirds, I use 1/4" X 5/16" or 1/4" X 3/8" hardwood. I cut my spars from spruce 2 X 4s, as I am not a fan of balsa spars and most of basswood spars you get are crooked. If you can find some straight basswood spars in this dimension, you should be fine, just be sure to put shear webs in to make the D type construction.
I have a scratch built FW with 54" wing span using 1/4" X 1/4 spars that has been put thru some severe muanuvers, plus it flies about 80+ MPH. Never had a wing fold yet, so I must be over building, but I like it that way. Good luck.