Here is my car with 8-1/2" rear centerlines with a 5" backspacing. Here it is again with the new Ralleys, 8x15 Rears with a 4.5" backspacing, 7x15 fronts with a 4" backspacing. Both have 275x60x15 BFG RadialT/As and 235/70/15s on the front. The car is not jacked up at all. If I was to get the 10" ralleys, get the 5-1/2" backspacing with the same tire. Only use the 5" backspacing if you put the early B-Body 8+3/4 in it with offset shackles and front spring mounts. The wrong backspacing will get the tires to eat up the bottoms of your quarter panels, it'll be jacked up with longer shackles or air shocks, and you can't speed around corners like I do. If you put a standard 15x7 up front, with the standard 3-5/16" backspacing, your front tires will rub the fenders, when you go around turns and hit bumps at the same time. (like turning into a driveway with a street curb).
Rob