That looks like a cool rock, I would use it . The rock we put in was about 5ft long and came to a point. If you search my build, you can see how they installed it. It sticks out about 8-10 inches into the pool.
Just found your build thread. Awesome pool. Rock is the perfect size. I found a place that is selling them by the pallet for ~$100-$140 and weighing 2500 lbs. So the rock is cheap good rock...but getting it here -_-