I think I have a 1 inch or 11/4 inch spacers on the rear of mine with 17 inch zo6's. Not a definite answer but I hope it helps.
I have 18x9.5 C5 wagon wheels. Not sure of backspacing, but the fronts line up fine w/o any adapters/spacers. The rear needs 1.25" adapters. I have 1" adapters currently and a sharp eye can tell the tires are tucked under a little too much. Assuming our wheels have the same offset, I recommend 1.25" for the rear.