Will a 15x10 wheel with 7.25 backspace fit a 1970 Dart with 8.25 Rear

You'll need a mini tub if you want to tuck tire, and a spring relo forsure no matter what if you dont want the 80's jacked up, wheels sticking out beyond the fender look. That wheel and backspacing should work pretty good, but I'd run a 325/50/15 tire.

That car in your post has also had the axle narrowed almost guarantee. Keep in mind if you want the "deep dish" look like the car in your post as mentioned already, you'll be narrowing your axle too. But with a 7" backspace rim that wont be neccessary. Just depends on the the look your after.