Clinton, I don't think that species-to-species is a critical consideration. Remember that your braces will be at 90 degrees to the grain of the back anyway, so expansion / contraction rates won't be a concern.
Well, let me say this: no more a concern with mahog than with spruce. Those braces won't be getting shorter / longer with the expansion / contraction of the back.
I have been taking a cue from Ervin Somogyi lately, who--as of our last conversation--liked to keep the species the same. And I mean by that the same species of the internal wood. So, if he used mahog (or Spanish cedar) for linings, he would use mahog braces, head block, etc. A visual thing. Everything matched everything else. Presents a cleaner appearance for those who like to peek through the soundhole.
Here's one I'm doing in African Blackwood:
It uses Spanish cedar for the linings, Hond. mahogany for everything else. Oh yes, the seam reinforcement strip is French polished.
Steve
