Going to cut axle tubes

Narrowing your rear only needs to be done if the tire wheel combination is wider than you can fit in your fender with out rubbing the outer fender, and you don't want to cut wheel well opening or you don't want tires sticking out. Even then you may still need to trim fender well opening.
It's all a matter of how wide your fender well is, the back spacing of the wheel and the tire diameter and section width.