I ordered my wheels with the backspacing that I wanted (18x12 with 6" BS), mounted the 335's and placed them up inside the mini-tubbed wheelhousing. Once I was happy with where they were, I measured the width from wheel mounting surface to wheel mounting surface. Subtract the rotor thickness for each side, and I had the overall width of the rear end, flange to flange.
As for the rear, I went with a 9" with an aluminum housing from Currie. I went with 31 spline axles and a nodular 9+ race case. You can certainly go larger than 31 spline axles, but then you really start running out of options for limited slip.
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