A 10:1 scarf joint will retain at least equal structural integrity across the span as long as the grain is the same direction between the two pieces. (Read: it doesn't matter where)
Edit: it doesn't matter in terms of strength, often the joint is stronger than the surrounding wood which changes properties of flexibility and stiffness, for these reasons some would choose to centralize the "hard spot" though in this case without finite element analysis of the completed spar structure it'd be hard to gauge what differences if any may exist.
There's several species of wood suitable to build with, I'd encourage you to look at what's in you're area if possible. Ash and Doug Fir are sometimes available in the lengths needed at the big box stores, just gotta dig for the right pieces.