I'm thinking some rotational play is to be considered normal in this situation. If the back functions properly & is light tight, it should be good. You may be getting involved in a quest for perfection that may prove fruitless.
If you do follow through, Check the lip & groove that hold body & back in alignment. Your body should be OK since there's less play on the original back. The lip on the new back may be worn allowing the play, but since it's rotational, not fore & aft should have no effect on picture quality.