I am not sure what you mean by 3.5" backspacing. Is this the distance from the centre of the rim outward to the edge of the rim. on the motor side rather than the outside
You're pretty close. The backspace dimension is on the inside (motor side) of the wheel, but it is the measurement from the mounting surface where the wheel bolts to the hub to the edge of the rim.
On the wandering, I would say it's most likely the steering box is out of adjustment. There's an adjustment screw on the side of the steering box to take care of that. Another possible cause would be paly in the tie rods, especially the original rebuildable style. Those can be adjusted for play as well, you will need a drag link socket to do this.
