Debating on not mini tubbing

With the stock wheel tubs the widest tire you'll be able to run out back is a 275, even with the spring relocated to the frame. The stock quarter lip to the inner wheel tub is only about 12" on a Dart. With room for clearance that only gives you about 11" for the section width at the most, which pretty much means a 275 wide tire is the best you can do.

As far as the width of the rear axle goes, figure out what wheels you want to run first, then use that backspace to figure out the axle width.