Not sure if you mean stretch them out thus widening the "hips" of the car, or making the opening longer front to back. I'm guessing front to back.

One way is to cut out the entire wheel opening lip area..............picture a rectangle starting just in front of the opening, ending just past it, relatively close across the top. If what serves the intent is to spread your stretch evenly from the centerline of the axle then you cut the rectangle down the middle. Trim 2" from the front of the new "hole" in the quarter panel, and 2" from the back. If you use the cut out piece as a template, or are carefull about how you trim, the joint should be very tight, or trim as necessary to ensure a tight seam. Then butt weld the sections back in. You'll have to form a 4" piece in the top center to match the "flair" shape, and/or rollunder. If you've trimmed close to the opening this should be a fairly small piece.