offset shackles (how big a tire)

The wheel wells on Darts and Barracudas are so different that you cant use one example on the other car.

My Barracuda had 255-60-15s on it with the stock spring location. 15X8.5" rim with 4-7/8 backspacing. After I moved the springs I had 325-50-15s on it in the stock wheel well.

My Dart has much less room. 235-60-15 is about as much as most guys get under them with out moving stuff.