What PARTS did Hurst and Chrysler put in the Super Stock Barracuda & Dart not in other factory cars

davescuda said

from the spring to the backing plate. If it was a b body rear it would have a lot more space.

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That picture above looks like an 8 3/4.
You must remember that the rear wheels where also modified. The centers were pushed out an extra 1.250 to account for the wider rear end. Chrysler new that the racers were going to tub and narrow the rear end.....they just wanted to make sure the racers had parts they can work with and modify on there own.
In that picture the rim has a lot of back spacing and a slick tire. Look how much "bubble" the tire has and that is why the tire is close to the spring.