On my car it looks like it would be 16 1/2” to 16 5/8” when using the “Second Generation” cutout from centerline of the axle to the most forward wheel well. I have the first generation cutout on my Dart and that measurement is 14 7/8”.
If I was building a race car or a car with custom wheels I always tell them to get the wheels and tires and then build the car around those wheels. You can always take my template and push it forward on the car and make it as big as you want. This way it will give you the factory shape that your looking for.