The "backspacing" spec will not be in any factory publications, as that spec tended to become popular in the '80s. It WILL vary with the rim width, too.
The older spec was "offset" which relates to the centerline of the wheel's total width relates to where the rear of the wheel touches the brake drum/rotor it attaches to. But measuring this spec is not as easy to do as measuring the backspacing of the wheel. ONLY thing is that many measure the rim width and backspacing from can be measured from the inner edge of the rim, rather than the "bead surface" of the rim . . . which can affect the measurement significantly.
The typical backspacing for a factory/repro 15x7 wheel is 4.25". The space between the wheel mounting surface and the outer edge of the wheels tire's bead-contacting surface. Place a straightedge on the outer bead surface (NOT the edge of the flange!), then measure the distance from that bead surface to where the backside of the wheel touches the brake drum/rotor.
As mentioned, the backspacing will vary with the total rim width, whereas the "offset" can remain the same. A "0 offset" wheel of 7" rim width would have a backspacing of 3.5", but many OEM wheels in that width are 4.25" backspacing, from what I've found. A 6" rim would be 3.00" backspacing at "0 offset". It can all be pretty simple, but also can get confusing, too.
My information on the "typical 15x7" wheel came from a Z/28 website with that information on it. Many Chevy wheels had more backspacing per rim width than many Chrysler wheels tended to, by observation. Not sure about Ford-spec wheels, though. Many people still measure rim width to the outside edges of the rim's flange, rather than "between the bead surfaces", so the whole backspacing situation can easily be mis-stated, as a result.
Possibly the best measure of "Mopar Backspacing" for 15x7 wheels would be the '79 LeBaron 15x7 W23-style wheels? Or the more prevalent 15x6.5" '76 Charger SE Rallye wheels? Or any of the "cop car" wheels (15x6.5 of 15x7"? Anybody have any spare wheels they could measure and post pictures of ?
Thanks,
CBODY67