Edward,
Thanks for the compliment.
Those are two 4" X 30' towstraps with a 20,000 pound rating, and an eye on each end. We just attached the chain through the eyes. So, the hull spun/slid in the slings. The cardboard served two purposes, to protect the straps and to also act as a brake. Cardboard is like a natural brake lining material. If you ever need to haul something heavy in a truck with a slick cargo surface, put cardboard under it, it will stay put.
We backed in the shop afterwards and burned out the strongback with the plasma arc. Only took an hour or so.
I was amazed the flip went as well as it did. I had plenty of time to think about how to do it. The method used seemed the most logical. That same method could be done by simply suspending each sling from above, high enough to allow for the spin. Then use one chainfall/winch to spin it around.
I thought about using an axle above, rolling the slings over it, but there would be no way to control the speed. The bottom being much heavier than the top, it would have whipped dangerously at some point. I didn't want to see that
-Ron