Skinny Wheel/Tire Fitment 68 Barracuda

A 5'' wheel will have a 2.5'' backspace 6'' usually has a 3.25 backspace.
Offset can be,usually negative for our old cars. I believe offset and backspace are one in the same as it is being measured from the centerline of the rim measured on the back side of the rim's surface lip to mounting face.
If that makes sense.