Rear wheel backspacing is the distance between the inboard edge of the wheel and the mounting surface of the wheel that bolts against the axle hub. It is measured by placing the wheel face down on a flat surface; lay a straight edge across the inboard edge of the wheel, and measure from the wheel mounting surface to the straight edge with a ruler.
On my 69, I require a 4.5" backspacing for my wheels...
Note this handy visual reference. On the Picture, "Back Spacing" is referred to as the "Rear Spacing"