Backspacing And Rearend Length Question

Well just looking at the math, it seems you should use wheels with 1 inch extra backspacing than you would use with your stock axle. So theoretically, if you'd use a zero offset with 4.5 backspacing on the stock axle, you'd need a 1 inch positive offset putting it at 5.5 inch backspacing. To make up for the extra inch wider the 9 inch sticks out on each side. Not sure what the stock BS and offset is on a 68. But for a 66, those wheels would be as I described above.