There's really no need to go to a 15x8 in the back is the thing. A 245/60/15 is the largest tire you can safely plan on getting back there unless you're willing to trim/roll the quarter lips, and those work great on a 15x7. I mean, you can run a 15x8, but keep in mind that you probably have about 11" of space to work with from the spring to the quarter. A 1/2" on either side needs to be clearance, so that only gives you 10" to work with, and a 15x8 is 9" wide outside lip to outside lip.
With a SBP 7.25 and a 15x7 you should be looking for a 4" to 4.25" backspace, but measuring would be your best bet. Take the rim off and measure the distance between the wheel mounting surface and the springs, and then the WMS to the quarter lip. A tape measure and a square work fine for that, just remember you want to measure the horizontal distance from the WMS to the spring and quarter, not the distance on an angle. Which is why a square works well.