offset shackles (how big a tire)

Dart abd Barracuda will be way different from each other. The Offset kit should move the spring just enough allow you to use the entire wheel well. You will not get a 10" rim in there with out jacking the rear up sky high. Other wise it will hit the quarter panel.

You should be able to get a 15X8" rim with 5.5" backspacing in it though. Look for a tire with around 10" of section width. That should be 9" of tread width.

If you want more you will need to mini tub it.