Sounds like the board is either plain-sawn or rift-sawn, probably the former from your description of the wavbiness on the wide side. You could make bows out of the narrow side, I would make them pretty long though, like 68" for a 28" draw, and I wouldn't make them over 50lb either. 50mm wide would be better as was said already.
As for backing, it is tough to tell if your board has violations of the fibers. You really have to evaluate that yourself, or provide us with some pictures. You know the grain is running straight as far as the narrow side, but waviness on the wide side suggests that the grain is running out some there. So I would suggest backing the bows to be safe. Linen isn't too heavy and is good insurance.