I was once a piano tech.
I've rarely seen spruce in a piano that I'd want to use. Generally, the standards are lower and the grain used in pianos is much wider than would be acceptable in a guitar.
On a grand, tops are usually either Spanish Cedar or Mahogany, veneered with whatever matches the rest of the case. By the time the veneer is removed and the core cleaned up, you might have some usable wood that's 1/2" thick, but it won't necessarily be quarter sawn.
The case is often oak of some kind that's been veneered. That's the bulk of the solid wood in a piano but it's been lam'ed up and not usable, in my opinion, for guitar construction.
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