Richard,

I'm trying to wrap my mind around 40" butt end logs for sills.

What kind of wood, with what kind of taper?

I think you'd faint if you saw some of the hewn timbers on some log buildings in the Alps. Think 18 inch faces, 6 inches wide. And they got 2 out of 1 log (cut a big one and sawed in in half)
Massive logs yielded those. But I saw no shortage of 4 foot spruce (which has very little taper over a tremendous length.
Add to that, the early builders obsessed about having uninterrupted timbers, even though it actually isnt important at all in this case. (tightly fitted log walls erected on a solid masonry socket)

I like your log dogs. Simple, but effective.