If you think real hard about it, primitive man didn't build primitive bows. He built state of the art equipment that was as efficient as possible for his time. So cutting a shelf in is just another way to make things that much more efficient.
An 1 1/4" width is pretty common for Osage bows between 50-70# depending on the density of the wood, but for the weight you're wanting, I'd go at a minimum of 1 1/2" for the width and 66-68" for the length. I would want to leave at a minimum 3/4" of wood at the arrow pass/window for that weight. Maybe 1 3/4" handle depth. Go for it. Good luck..........Art B