Now here comes the moment of no turning back. Cutting the back. What I did was to place the motherboard where I wanted it. Then I marked the width on the back. I knew that the motherboard would be sitting above the power and reset switch. I measured where the top of those switches stopped then made my marks.
Now cut the back. Once you have it all cut you can set the motherboard in and measure how high the back plug ins go past the dark grey portion. Mark it on the light grey portion. Make your cut. You can see where I marked where I needed to cut. You can also see I didn't cut for the back plate but only what I needed. That way the fan will have a place to blow out all that hot air.
Hard parts over. Get a beverage of your choice and celebrate. Mine is a Vanilla Porter I brewed up.