Question on backspacing on rear wheels

Sure. Pictures show the spacing to the spring. As you can see, plenty of room to the quarter still. But only 1/2" to the spring, if that.

With a 15x8 you can run 4.5" of backspace without issue, maybe even 5" because of the way that the backspace measurement works, a 15x8" with a 5" backspace should end up about .3" closer to the springs than the 15x7" with 4.25" spacing. But 5.5" backspace would move the tires in .8" from where mine are, and as you can see there isn't that much room. You'd need spacers to manage that.