Typically the rear end manufacturers recommend at least 1" of clearance to the inside for a street car. There is another way to double check this, we know the dimension of the frame rails (outside to outside) add 2" to that number for clearance (1" per side) then Take a straight edge across the back of the tire and measure your total backspacing (wheel & tire). Add that (X2) to the previous measurement and see what you get. If you get 54.75" then that's it.
Theoretically 38" wide rails + 2" clearance = 40" + (2) 7.375"BS = 54.75" Give or take...