Easiest way is with a staight edge and a tape measure put the straight edge on the hub face and measure to spring and then to fender. You can figure out your offset or backspace that way too depending on your rim choice.
I fitted a 2756015 on a '71 Dart with SS springs and a 15X7 steel wheel. I'm not sure of backspacing though. I could get a finger between tire & outer wheel lip. Can't remember clearance between tire & spring. It did clear but was close. Ended up using a 2556015.