Tim:
I think one thing we can learn from the pictures above is that these cars were all a little different in this department at any given time in their useful lifetimes. I would suggest you just do what you think looks "good" to your eye. For me, it would be something like this:
I would cut it per the red line visible in this quick drawing. Personally, I would think that as long as the pipe is cut flush with or a bit inboard of the rear wheel well contour line, you will be in a good place visually. Just my humble opinion.
- Allen.