Yeah that's about what stock should be.
255's are out. You only have 10.75" to work with then on the "tight" side. Even assuming a 255/60/15 is only a 10" section width when it's on a 15x7" that isn't going to work. If you gave the quarter a 1/2" of clearance you'd only have a 1/4" to the springs, and that's not enough.
I would say a 245/60/15 is as big as you can go unless you trim the quarter lip or install 1/2" spring offsets. And 15x7's with 4.25" of backspace won't work. The math to center everything up on the side with less clearance gives you a 15x7 with 4.65" of backspace, so, what I would probably do is order a set of 15x7's with a 4.75" backspace and a set of 1/8" spacers. It will be right about 3/8" to the springs with that backspace, you can play with the 1/8" spacers if it's too tight on the springs.