Those were made for rapid installation. The guys could quickly remove the sling once the engine and transmission assembly was set in. It's especially important when you only have a minute or less to get it done. Of course, having to set it in a bare frame makes it easier. Not to put too fine a point on it, but how many times are you going to remove and replace your engine?
Or are you just wanting to make it look like factory?
Just a note- I think that one of the most amazing things I've ever witnessed in vehicle work was watching guys putting a front bumper on in less than a minute.
Without scratching the paint. 