Having 15x3.5 wheels on the front with anything much bigger than a 15x6 wheel will most likely give it lopsided grip and it won't handle all that great to begin with. The 3.5 wheel is more or less so people can specifically run bigs and smalls which isn't a handling thing, it's mostly for straight-line grip for drag strips. So a 15x7 or a 15x8 really isn't going to matter, it'll all be the same in the scheme of things.
Now if you're looking to use the 15x7 in place of the 15x3.5 on the front, that's a different story altogether. I guess I'm under the impression that you're asking about rear fitment. I don't really have any experience with a wheel/tire on the front that's much wider than a stock width. I do know that your overall diameter will be limited, and I do know that if your backspacing isn't right the wheel will run into the upper control arm, but I don't know if there's a 7 inch wide wheel that works without rubbing the control arm AND doesn't stick out so far that the tires have clearance issues with the fender. If you can't find anyone who has anything like you're looking to do in the wheel and tire thread, you might need to pick up one of the tire tools from Summit or Jeg's or something.