What length rear end fits a stock plymouth Barracuda?

Does anyone know offhand what length rear end fits a stock Plymouth Barracuda with unmodified wheel wells? I have a stock 8.25 from a 1973 Dart 340 car. It has the large bolt pattern and I moved the springs in about .8" to get more tire room. Do I need to change my length from stock to get the maximum amount of tire? Rear is about 56" now ( stock dart a body ). Can I get 255/60/15's on back, or more? Has anyone with a stock 1968 barracuda done the work to figure this out. Just trying to save myself some time.