Backspacing question

I'm in the process of measuring for new drag wheels and tires on my 69 Dart. The car has ralley wheels on it now and the guy I bought it from said that the rear wheels are wider than stock from a place that does custom wheels. I wanted to know the backspacing on them so I have an idea what I need and I meaure 6" of backspacing?? Why is there so much backspacing? Im looking to get Weld Pro Stars in 15x7 with 4.5" backspacing. But now I dont know what to do. I do know the 8 3/4 was out of a 66 B-body and was told it was shortend to fit the A-body. Any help would be appreciated. Thanks!