I'd wait until the car is back on it's wheels fully loaded, then you can get the exact backspace you need to maximize track, and keep the tire just inside the flare. You might be able to use your old wheels and some washers to figure it out.
If you keep the same 5" backspace (which is just about perfect with coil overs), you 10" wheels should stick out another 2" from where they were before the flares. It might be just about right with the 3" flare, but I would wait and order your wheels when you know for sure.
Not to hyjack the thread, but what cage did you end up going with, custom?