Take the rear out.

Obtain the wheel/tire combination you intend to use. Set it all up under the car with the wheels/tires exactly where you want them at ride height.

Measure inside of wheel axle flange to inside of wheel axle flange.

Subtract brake hat or drum thickness, and axle stick out. Axle stick out can change with housing end type. Result is housing width.

Anything else is guessing.