I'd probably do some measurements and see how centered your rear sub-frame and axle to your body are. On my car they were about 1/8" offset to the driver's side. From my understanding this is not uncommon and can affect the wheel sizes and offsets.
I got around my offset by rolling the rear fender lips especially on the driver's side.
I used a 17x12 with 8.25 BS on a 335/35/17 Michelin PS/2 Tire. But I didn't narrow my rear end at all kept it the standard width for a 69 Camaro (didn't want the deep dish went for more of a sleeper look )
Hope this helps