generally speaking, one side of the prop is all you need to grab to start tighten the nut, as ChuckW mentioned the nut that you are using is not the apprpiate for this setup.
The shaft of your engine should have a threaded portion and an unthreaded portion just where the flywheel meets the shaft. Your dubro backplate spinner should sit flush with the flywheel, there are some dents at the flywheel as well as the backplate spinner and that is enough to avoid the engine from turning when you are tighten the nut.
I would change the nut and try to find the right one.