Cutting the impeller off is a last resort unless you are in a big hurry, but I have had to do it. I have a set of tapered pry bars I use to wedge behind the impeller and soak it with oil and let it set. Go back later and hit the pry bars, wedging them in farther and soak it again in oil, doing this over a few days. But you have to think about what you are wedging up against, if it is the engine, you can crack the engine case. Heating it with a torch is another way for getting it off, but safety is the word here, be careful if you have a torch..... If you are not going to take the impeller off, drilling the hole in the crack is a good way of stopping the crack, but I have seen cracks migrate pass the drill hole, just keep an eye on it.