15x8's with a 4.5" backspace are 0 offset, you'll have even less space to the fender up front. 15x7's with a 4.25" backspace (OE spec for cop rims) are +6 offset. 225's are about as wide as you can go up up front with those specs unless you've got a barracuda or you want to roll your fenders, then maybe a 235 but that's pushing it.
There's really no point in running 15x8's up front. You can run a 225 just fine on 15x7's and you can use OE offsets to do it. To run even a 225 on a 15x8 you'd need 4.75" of backspace, and that may give you grief on tie rod, brake and ball joint clearance. You won't be able to get much more backspace than that to work, so, you can't run more tire anyway.
To go to 8" wide rims up front and make it worthwhile you need at least 5.25" of backspace, and you need 17's to do that.
