It depends on how low you sit the car. If you don't drop it as low as I did, then you can get by with less offset.
I'm guessing by your 18x10.5 in the rear, you're gonna have to sit the car pretty high to keep it from rubbing. Depending on what B body rear you have, an 18x10.5 with 15mm offset is going to sit ~1.5" further out than where my tires currently sit. In other words, you're gonna rub the outer fender well in the back when you hit bumps.