need alittle help with rear axle width , im going to build my new car with a 21" shoulder hoop , the rear wheels im going to run are 12" wide with a 6" back spacing . the tires im thinking of using are goodyear d5 31x13.0-15 tred width is 12.8 im if figuring correctly , outside of hoop will be 24" 2 inches of tire clearance from hoop to sidewall , 6" back spacing , 3/8" for rotor thickness ... im at a 39" wide axle , from outside of axle to outside of axle ..... does this measuremet of 39" sound correct ?