Not good, you would be installing a skinny 4 inch x 63 inch piece with little under-support other than the cabinet walls. You would have to work in the epoxy over the whole length and the seem would be in the most obvious place, full width directly on top of an island. I dont even think any granite company would do this if you asked them to. But they might agree to cut up the old slab for you to reuse somewhere else.