What width rear end for Duster Street Strip stance?

Id bet the red car has mini tubs.

The way i did mine was pick out the rear wheels and tires. Install mini tubs. Lower car over new wheels with tires mounted. Center wheels and tires in tubs. Measure from inside to inside of wheels.
Subtract for break off set.

That is the width of your axle.

There realy is no set number as folks use different back spaceing. Some like a lot of back space others dont.