The only measurement that means anything is Wheel Mount Surface(WMS)to WMS. My 18x12 5.5" bs are mounted on a 57" WMS to WMS Moser 9" full floater housing. If you narrow the housing consider changing to Big Ford ends they use a much larger bearing. Or you could install a full floater hub kit from Moser. Since you need to narrow the rear housing anyway, just be sure to look at all your options first.
