The outer stem should flow smoothly into the keel. Ideally it - both external stems & keel - would look like one piece of wood.
I was thinking about my advice to you above. I forgot to mention something: learn how to make good scarf cuts. First, steam/bend the external stem, and then trim it into shape (more on that below...). Now, you'll need this stem to extend into the flat of the canoe into the keel area and lie flat to the bottom of the canoe like a keel would. If it does then congratulations, you're a far better steamer than I. If you find it easier, steam bend additional wood and then scarf cut & glue (Titebond III) piece(s) to get the needed shape. THEN scarf cut & glue the keel into place. Once this joint is dry you can make the 45deg cut I describe above: cut at 45deg from the surface that'll lie against the canoe and stop cutting about 1/8" from the other surface. Then turn the part over & cut through to the 45deg cut you just made. That'll give you a perfect fitting joint. Repeat with the other external stem except the 45deg cut will need to be accurate to cause both stems to be the right distance apart to fit the canoe properly. If the distance is wrong you can always recut or scarf-cut additional wood to the keel. Come to think of it, a more acute cut angle would be better - say closer to 30deg. Get the idea? And it's probably better to do much of this fiddley work after canvassing but before painting.
As for steam bending, I think you'll find that the wood will need to be square shaped in profile to bend without cracking. So you'll likely need to trim the width since the external stems I've seen are all a rather steep V shape - maybe 7/8"x 3/4" with 3/8" flat on the outer edge. Then you'll need to flare the height down to match the keel height.
As for caulk, make absolutely certain whatever you use contains NO SILICON. Once silicon touches a surface paint won't stick to it, and you'll have a devil of a time removing it. As for caulk type, look at my posting a few days ago for runny caulk:
http://forums.wcha.org/showthread.php?13595-Bedding-compound-too-runny
And oak or ash is a great wood to use for steaming. They say steam for 1 hour per inch - I steam for 75 to 90 minutes.
Ya know... just typing this out makes me realize how much work is involved. I'm gonna be selling this Rainbow (I restore too many canoes to keep 'em all) - makes me think I'm asking too little for it!