Stock Camaro would be 9.5” with a 5.5” backspacing.
Subtract the width of the new rear end from the original. Divide the difference by 2. Subtract that number from the 5.5” backspacing.
60-57.75=2.25
2.25/2=1.125
5.5-1.125=4.375 backspacing
Don’t forget to adjust for any offset introduced by non stock brakes.
If you want to give yourself a little wiggle room add .25” to the backspacing so you can adjust with a small spacer.
Don