backspacing
Not necessarily. That number, the section width, changes by tire manufacturer, model of tire, etc. For example, BFG T/A's in a 255/60/15 (the only tire that tirerack sells in that size) list a 10.2" section width. With 255's on a 70+ Dart that makes a big difference, because on most 70+ Darts with the stock spring locations there's only about 11" from the inside of the quarter to the spring. That extra .2" basically means your tire has to be perfectly located to not rub on something, and even then ride height will come into play.
On my '71 with an 8 3/4, BBP axles and the stock spring locations I have 6" from the spring to the wheel mounting face and 5" from the wheel mounting face to the quarter. That's with axles intended for the later BBP brakes.
With axles using the A-body SBP offset and redrilled brakes it would probably be closer to 5.75" from the wheel mount to the spring and 5.25 from the wheel mount to the quarter. That's going the make a +12 offset a little too close to the springs. Remember, backspace is measured to the inside lip of the rim. So a 15x8" rim is actually 9" wide, and if it's got a +12 offset, and therefore has a 5" backspace, the outer edge of the rim is actually 5.5" from the mounting face. The tire is another 1/2" inside of that (it's at least 10" wide), so you're at 6" from the edge of the tire to the wheel mount. Actually 6.1" if the section width still turns out to be 10.2. With an A-body SBP axle offset +6 is more like what you need, which on a 15x8 is 4.75" of backspace. That should put a set of 255/60/15's right at the spring, and you can adjust your spring clearance with a small spacer to maximize your tire to quarter clearance, which needs more space than the spring.
If you have the brakes on your car now, you can use a carpenter's square to measure from the face of the drum back to the spring and out to the quarter and confirm your numbers. These cars are all a little different, and they're frequently not even the same side to side. My '71 Dart has an extra 1/8" of clearance on the passenger side than it does on the driver's side.