The wear ring should be a tight fit, if it's an OEM you should have to drive it in with a 2x4 and a hammer or a press if you have one. If it's not a snug fit the wear ring is most likely an aftermarket. A trick I have used is put it in the freezer overnight, it will shrink slightly and make it easier to install.
Added: After reading your question I'm not sure I answered your question, but considering the wear ring is not under compression it's going to be hard to tell how the impeller will fit, personally I wouldn't worry about it skat trak has been making impellers for more than a few years and I'm sure it will fit.
Lou