using 1x6 for framing exterior walls
Buy the 10" boards and rip them in half for two 5" studs for the least amount of waste, or to 3-1/2" if you want readily available windows and doors to fit in your openings which is going to be your next problem. Since the wood is spruce, follow IRC building codes for 2x4 SPF load bearing/exterior walls. At 8' tall and 16" on center spacing you can support most roofs, but don't support both a floor and a roof on it without asking us or reading up on it first. Use closed cell sill sealer to separate the bottom plate from direct contact with concrete/masonry, or just find pressure treated lumber for the bottom plate instead.