I've been through the wheel deal with tires and what back spacing to use with 1/2 inch clearance on both the front and back side of the tires and wheels
Are you going to leave the stock wheel wells and outer 1/4 panels stock? What year Duster?
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The stock 8 3/4 housings for the Duster and other A bodies is right at 51 1/2 inches wide from the outside of the wheel bearing flange that the brake drum fits against to the other side, how wide is yours now ?