I know, i know - doesn't seem realistic - but the stiffness, strength numbers all look quite good. In fact, beats out White pine in all but Compressive strength (4800psi pine vs 4730 psi basswood). Also easily obtainable in long lengths, with very straight and clear grain. Biggest issue i see from the surface is the grain is very light and it may be difficult to accurately grade the material - counting rings per inch and runout on wood with very faint grain. Seams to me would be a good option for rib stock and secondary members. I might grab a few pieces and run some tests compared to Northern white pine.

Consult report 354 aircraft wood properties and selection by forest product laboratory. Sounds like similar to pine but I always thought pine was flying a little bit on the margin of safety.  And not all pines are the same.  My plans called for ponderosa pine which is a stronger pine.  If you have basswood available you can increase its size to match that of sitka spruce without too much weight penalty.  Hard woods don't necessarily require the same number of rings per inch as soft wood( report 354) as they can achieve full strength at faster growth. There are you tube videos on testing aircraft wood for strength to make sure your basswood is up to the task. You also should be testing the specific gravity of your wood. This tells you how much wood fiber is present...again, you tube videos.

Wing ribs were often made of basswood in days past. Without increasing the size. For longerons and spar caps I think I would increase the size to the strength of spruce...what ever the percentage difference in strength is between the two. But you absolutely can use basswood if you are willing to learn a little. Report 354 is all about alternative woods for aircraft.  Also, basswood is actually lighter in weight than sitka spruce so increasing the dimension slightly is not going to amount to much. If you wish more help on this subject you can email me and I can discuss it with you further.

Alex welcome to the ETLB. How is the wood procurement going? We'd love to see any progress pictures/reports.