Any additional width to the rim will go out not in.
5.5" backspacing is the distance from the innermost edge of the wheel to the mounting surface, (like if you laid it good side up on the driveway, and measured up to the mounting surface). So a 8.5" width rim will have the same spindle clearance as an 8" rim if they have the same backspacing, (and are the same design). The 8.5" rim will be a little more deep-dish.
By going too wide, or too small of a backspace, the tire can rub at the pinch weld, when near full steering lock.