Axle Length

It all depends on the offset of the wheels and the section width (actual sidewall width) of the tires you will be using making sure the tires clear the outer fenders. For better accuracy, it would help to have the tires mounted up and set inside the wheelwells after you cut out the OEM sheetmetal.

You need to be sure of the rear measurement because wheel and tire manufacturers usually don't take them back after they are mounted if the rear is incorrectly cut first.

When you mock up the tires and wheels, you can contact the axle manufacturer you'll be ordering from and give them the measurement. Then they can tell you how wide just the housing needs to be based on the distances between the axle flanges and housing flanges on a 9" as well as the axle lengths for it..