I have always found it very curious that the Very last Timber Element to have left the Residential Building in the North East -- the Timber Sill -- is also the very last Element Modern Timber Framers seek to incorporate into their Frames.

Beyond merely providing a place for the Post to land, a Sill acts to spread the Point Loads, And, to help the Frame survive the erosion, and, sometimes, the outright collapse of sections of the Foundation.

In the Modern House the Foundation is often one of the more temporary aspects.

In general, Softwoods can be substituted for Hardwoods in most every case, (including braces) however, Dimensions do need to be refigured. I don't remember what Spans Jack's Design addresses or what Sizes he calls for, and why he specs Softwood for Horizontal members and Harwoods for Vertical, rather than vice versa.