I'm afraid you have more problems than you realize. The shower drain should be 2", and the toilet requires at least 3". Yours may be 3" or 4". The location of the two drains as related to the walls are not the same. In short, your entire plumbing will have to be redone. As often mentioned by pros on this BBS, nothing is impossible...given enough time and enough money. This job will require plenty of both.