Personally, I'd not put that much work into a questionable stave. Sinew would probably save it but it might not. Plus, a 64" bow is too long for sinew in my opinion. But, if you want to try it, I'd glue the cracks with superglue.
I'd use TB3 with the sinew. Thin it a bit with water and then put a thin layer on the bow's back. Soak the sinew in warm water, blot it dry and then run it through the thinned glue. Lay it on. I let it dry for an hour or so and then run another course. Three layers sound good to me.
You should let it dry for a week or so and it should add at least 10 pounds. I always stop the sinew about 6" from the tips to minimize weight out there.