I measured a 71 Duster body with a stock leaf and 8 1/4 rear and I get 21" from the mounting plate that the front leaf spring hanger bolts to to the top middle of the axle tube and I get 21 inches, with the car on jack stands and the suspension hanging down, no shocks. But I'd not go with that - put the tire/rim combo in the car where it gives you the clearance you need and work from that, and work out any changes needed to the drive shaft length, shock mounts, fender trimming, etc. Most Swinger bodies need a bit of trimming on the front edge of the lower qtr for tire clearance when the rear is in the stock location. Some guys resort to adding spacers between the front hanger and the body mount. See the tech pages on BigBlockDart.com for a step by step to show trimming one.