What you'll need is a 2 foot level,jack stands,and a tape measure.
Set the car up level at ride heigth with 4 jackstands under the suspenion.
Pull off the rear wheels.
Take your 2 foot level and place it against your drum.
Measure inward to the shollowest part off the inside wheel housing.
Wright down this measurment.
Measure outward from your 2 foot level to your wheel lip.
Wright down this measurement
You now know what wheel/backspacing will fit your car.