How strong is aircraft-grade spruce?
My recollection of the Modulus of Rupture is about 6000 psi for aircraft graded sitka, based on the maximum allowed grain runout, and a maximum of around 10000 for straight grain.  But you should be able to find everything you need in the old Forest Products Laboratory study at this google search link.

The key weakness in spruce (and wood in general) used for wing spars I've learned over the years is the propensity for dormant failures in compression, typically following a wing tip strike on a strut braced airplane, that places massive compression stress in the wood above the strut attach bolt. 

It collapses the cell walls in that area, evidenced by only a tiny line across the grain, that may or may not even be inspectable, and which drops the wood's tensile strength in the affected area to a fraction of normal.  

Dormant, because you don't find out about the problem until some day when the wing is loaded inverted for some reason, so that the failed section is loaded in tension, and oops there it goes.  For this reason I advise people to stay far away from an airplane with wood spars that's ever had a wingtip repair.