Debating on not mini tubbing

Currently I am doing a over the rear seat hump and trunk floor replacement along with inner wheel houses. I have moved the springs into the frame rails. The plan is to narrow the rear end also although I'm not sure how much yet.
If I don't minitub how much tire can I stuff into a 68 Dart wheel well?
How much would you shorten the rear end?? I have a c body rear to narrow.
Thanks,
Jeff