Traditional downriver designs like yours are longer and narrower than whitewater boats, but not necessarily as long as a touring boats. They were designed for river travel that's not as extreme as hard-core whitewater. They can be good for poking around in estuaries too, since they're easy to maneuver. Like many types of kayaks, downriver boats have become more specialized in recent years, meaning longer and narrower.

The main thing is that your boat should be a perfectly fine day-tripping or coastal play boat, once it's completed. You really can't tell how it will handle until you have it outfitted well enough that you can control it completely (foot pegs, backband/backrest and thigh braces). If it doesn't track well enough for your needs, Jay's idea of a temporary test keel is a good one. If you're feeling more ambitious, a retractable skeg is another option that provides more versatility that a fixed keel.

Regardless, this seems like a fun and interesting project.