Basswood is also known as Linden or "Linewood." It's really not a superior wood for arrows, but it'll work. It's a very popular carving wood, and is probably one of the best trees for making cordage. In fact, that's how's the name "linewood" came about. The underbark is superior for cordage and primitive rope making (i.e. lines).
Also, when I said that poplar averages 35-45# in the 5/16" diameter, it certainly doesn't mean that you can't get it in other spine weights. It's just after a couple hundred arrow shafts, that's what the average/majority ended up being.