What width wheel are you wanting to run? Backspace will vary depending on wheel width, as well as what offset is being used.
According to GM Heritage, the stock base wheel has the dimensions of 16 x 4 1/2", and an offset of 9/16". It does not specify positive or negative on the offset, however, I'm inclined to go with positive offset in this case. That taken into consideration, the backspace with the factory width wheel would be 2 13/16" if my math is correct.
