The rear wheels are 6" backspace, and yes, only offset leaf spring hangers. I also have stock width 8-3/4 and Cobra style discs. I also trimmed about 3/8-1/2" off the wheel lip. It's crazy how wide the factory lip is on these cars.
I've never messed with the early A-Bodies, but I have to bet backspacing requirements are no where near what a Duster body will take. My suggestion it to make you own measurements about 10 times to make sure. I used a carpenter's square to take all the measurements, then started my search online for off the shelf wheels that would work. Most tire manufacturers have their tire dimensions on their websites.