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

This answer is way over simplified but here goes.

Any wheel over 4.5" of backspacing is going to be at risk of the tire hitting the leaf spring on the rear of a stock "A" body depending on tire bulge.

If you relocate the leaf springs and mini tub the fenders giving yourself an extra 3" of room on the inside the wheels should work. IMO a 275/60 tire is a little small for a 10" wide wheel. Mini tub definitely required for a 10" wheel with 7.25" backspacing.

But what about the rear end and bolt pattern you changing that too? Anything can be made to work.:D

And that Dart in the picture doesn't have 7.25 back spacing probably more like 3.25" those wheels are deep. Totally different look than the wheels your talking about.