Well, no one else has answered so I'll have a go at it. All the canoes I work on are quite old, and all of them have had at least some amount of gap between planks, plus often some have lost a small amount of material along their edges. So honestly I don't pay too much attention to plank width when replacing them. I just don't want them to bind when they fit into place. And small width differences likely won't be noticeable because you've got the ribs breaking up the pattern every few inches.
Far more noticeable might be the color of the new planking once varnished. Old wood always darkens more then new wood when varnished - though I feel the difference is less noticeable in planking than with ribs. Anyway, you might want to stain the new wood to darken the color before taking in place. Experiment with a few scraps first.