need help with rear wheels and tires - backspacing? 15x?

I am going to put some draglites on my 87 bird. They are going to be 15's. skinnys on the front but I was wondering if someone could help me with the width and backapacing. I want them to tuck under the wheel lip because I have coilovers and want to lower the stance. Thanks Aubrey

rear wheels not fronts