315 or 325 tires on early a?

Not sure if this applies, but my catalog says that 315/60/15's have a tread width of 11 inches. My 30/10.5/15 Goodyear Eagle slicks have a tread width of 10.5 inches. My rear is narrowed to 50 inches flange to flange, and the inside offset of my Cragar Supertricks is 4 inches of a 10 inch rim.