Question about replacing 15x7 wheels with 15x8

Thank you for your hint. By the way my car is not Dart, I have the Blue 69 Barracuda fastback- in the attached pictuer.
However, I think I will go with 15x7 front, 15x8 rear - 4.25" of back spacing if it will fit like yours in the picture. although, I prefer the rear more wider so it will fill all space out the to edge of the fender; notice on your Green Barracuda, there is a bit of space between the outer edge of rear tire to edge of fender, if that space can be filled with wider wheel or wider tire, it will look nicer if I'm not mistaken! don't you think so?