3" is too big. You'll kill your low end torque. If you ditch the cats the flowmaster doesn't sound real good. I'm running no cats, 2. 5" from y pipe back with a SS MagnaFlow and 6" resonator. The exaust turns down over the rear axle. It's loud but sounds pretty good.

 

replace std muffler with magnaflow perfect rumble sounds as good as cat back system IMHO