If the impeller is behind the prop then put the prop shaft in a vice and tap on the housing carefully. it should bring the shaft and guts out as an assembly. watch for shims. make sure the drain plug is out so it wont create a suction.
Before you do that, be sure to remove the retaining ring AND the impeller housing lock pin... The lock pin keeps the impeller housing from rotating while the motor is in operation. if you try to force the housing out without removing this little pin first (#269 in the diagram below), you end up breaking out the impeller housing. Be careful, as Commander parts are hard to come by. http://www.chriscraftoutboards.com/commander_lower.html