I just finished a '70 Buick GS A-body with 17x10.5" front and 17x12.5" rears, so I think 8.5" wheels are no problem. However, 6+ inches of backspace definitely is a problem. A wheel with about 4.75" backspace is about what you want. A 8.5" wheel (measured from bead to bead) has about 9.5" width measured from outer lip to lip. The inner lip is where the backspace measurement is taken from down to the rotor/drum mounting surface. A 9.5" lip to lip wheel with 4.75" backspace places the mounting surface exactly in the center of the wheel. This is called 0 offset. This is perfect for the rear. A wheel with slightly more backspace is ideal for the front-maybe 5". BTW, please don't try the 10.5"/12.5" wheels at home without adult supervision!
Paul