8.25 rear in 69 dart change rear wheel back space?

If its an A-body rear the factory wheels should fit fine, best fit is probably 4 to 4.5" of backspace depending on the tire size. The factory wheels should be 4.25" I think, so they should be fine.

The A-body 7 1/4, 8 1/4 and 8 3/4 are all pretty much the same size. I've heard discussion about the BBP 7 1/4's being about a 1/4" wider on each side than the SBP 8 3/4's, my guess is that would be true for any of the SBP rear ends, the brakes and axle offsets were slightly different so going from SBP to BBP might make the rear a little wider. But for 15x7's I don't think you'll have any issues. If you're filling the wheel wells to the last 1/4" it might make a little difference. But if you're doing that then its different from car to car anyway.