Irish, the slitting arbor I made doesn't have a keyway. Come to think of it, the blades I bought didn't either. Skilsaws don't use them and the blade is a lot bigger. It helps that they can slip if something goes really wrong. I haven't had one slip yet.
If you are going to use factory nuts on an arbor, keep in mind that you should face them on the arbor. Factory stuff is rarely square or centered. Normal arbors have a turned shoulder with a boss sized to just fit the hole in the blade. As you can see in Grayone's picture, the blade is held by a cap with a SHCS threaded into the boss of the shaft. The cap has a counterbore in the hidden side to slip over the bit of the boss that extends through the thickness of the blade.