Every design feature is a trade-off... keels help the canoe go straighter, but they impede turning. So if you can't keep a canoe going straight enough without one, okay, add the keel. When running in quartering waves, they assist the waves in turning the boat dramatically from side to side, as the waves pass the boat, making it very difficult to maintain your heading. In a river , if you get turned somewhat sideways to the current, the current will push on the keel, and again turn your boat in ways you don't want it to, or push the entire canoe sideways, where you may not want it to go.
As MGC said, you can complete the work and leave the keel off, paddle it for a while, and then decide whether you want one. In the long run, you're better off learning to paddle without one, IMHO. Instruction is a good thing.
Except when you have outside stems, as Greg points out; outside stems, as applied to these older canoes, need keels, by design.