rear axle width?

[marty69212]

i'm building a 69 Camaro . I have 18x12 forgelines with 335 tires,6 inch back spacing , was told to go with 55 3/4 inch rear axle width . I have the tires ,wheels and brake rotors mocked under the car and im getting closer to 55.5 inches. does this sound right?want to get this right before I order a really expensive rear.thanks