Backspacing still doesn't make sense to me.

Jerry,
I can sympathize with you on this as I recently encountered similar issues on location of your measurements. I have a 72 dart swinger not a sport so I can only try to explain what I did. I mini-tubbed and did a spring relocation into my frame rails to gain space. I put in a new rear that had been cut down approx 3" per side or 6 overall. I jacked my car up (no wheels on) and dropped a plum bob from several places and marked those spots on the floor with a sharpie. Now I did this all over the place. First I did it from the center of the axle on the brake drum where the wheel would mount. Then on the other wheel the same thing and made a straight line connecting them so I had a good reference point. From the inside (dimple mark) just above the wheel lip, from the wheel lip itself, and from the outside edge of my leaf springs since this is the closest spot to rub on the inside. In my case I decided to hammer the dimples out of the outer fender well to make more room. So I dropped the plum bob from the closest contact point on the outside that I could find by dropping from several locations not just trusting by my eye. Mark all of these spots on your reference line and then using a scale you can get accurate measurements. Now here's the thing! Unless you trust someone to having or you are able to verify the exact measurements are the same to yours, your reference points will be different. Is your car using original leaf springs, super stock, or any other after markets? Mine is using super stock springs and I have a pinion snubber installed. So depending on how far your car will squat or wheel will go up into the wheel well will maybe let you get away with larger tires. Remember you want a little room for stretch when cornering etc. I compressed my rear until the pinion snubber would hit and then determined with my 28" tire how far up it would travel in the wheel well and then gave myself about 1/4 inch for flex. To give you an idea of my measurements. On the left side I needed 3.25" back spacing on an 8" rim and on the right I needed 3.5" bs. I went with 3.5" bs and then used a 1/4 spacer on the left side to even them out. I think using a plum bob is the most accurate and easiest way because I could see where exactly everything was at. If you want to fill your wheel wells and I mean stuff them to the biggest tire possible its the way to go. Now doing this, you should also know what tire you plan to use and take into consideration the sidewall because that is the widest area. It worked for me, hope it helps you out. If you have any questions let me know. The main purpose is to find the center of your wheel wells and then measure off where you need everything to fit. Is time consuming but is easy to get accurate.
This site should help with more detail and pictures...http://www.bigblockdart.com/techpages/wheelspacing.shtml
Adam