Help! How much space gained by rolling fender lips on '70 Dart?

I ordered wheel vintiques series 63 black steel wheels, and Cooper Cobras 255/60/r15's for the rear of my 1970 Dart. The 4.5 inch backspace seemed like it would be enough, and the inside is way good towards the springs, as they've been moved in...

But the outside edge of the Tire to the fender lip is like a quarter inch, and I haven't put the Drums on yet!!!! :banghead:

Now I am considering having the rear fender lips professionally rolled up flat, but how much room will that give me??? I have heard around half an inch, is that true???