Jon, you've done a superb job on this one!! You deserve to be pleased with yourself!
You will have to split the canvas to do the stems. Put that beautiful canoe frame upside down on some trestles, preferably tie the frame to the trestle so it does not move too much. Fold your canvas along the length to give yourself a centreline you can mark that with a marker of some description, drape your canvas over the whole thing and put your centreline on the keel, you can put a few tacks to hold the canvas in place along the keel, start one end with a couple of tacks then go to the other end, stretch the canvas and tack, then put a few tacks every foot or so. Then from the middle of one side you can pull the canvas tight and tack it along the gunnel, start in the middle, level with a tack on the keel, so you can stretch the canvas without distorting it too much: like that: tack 1 then 2 then 3 then 4 then 5, 1 being the middle. Make sense?
4 2 1 3 5
Then do the other side in similar fashion and so on along the whole length. You probably will need to adjust by removing some tacks every so often to pull creases out You can cut the canvas along the gunnells ( don't split the ends yet) but give yourself enough of it to be able to grip and pull tight. When its tacked along both sides, say every 4" you can start doing the ends. By then you should see how to finish the stems, cut the canvas , giving yourself enough to pull and tack one side along the stem, then the other side. Be careful that you may have problems with splitting the ply, try to stagger the tacks so they don't all end up in the middle layer of the ply. It may be worth your while to put a few screws through the tickess of the ply to prevent it from splitting. As they will have to stay there try using brass screws so they won't rust. Enjoy