So now we have our chain and our line. There is no shackle in the system. They make a pear shaped one that's perfect for that large swivel. The reason for using the rope to chain splice isn't just to eliminate the shackle, but also another weak link in the system, the thimble. Under load they have a tendency to rotate, and the throat of the thimble will chafe the splice.
Thimbles are quite large and impossible to get through the deck pipe.
There are two types of splices you can use to complete the rope to chain splice. The one you'll see the most is a back splice. Simpson Lawrence recommends this style for use in their windlasses. Theoretically the chain and line will feed below deck on the special wildcat furnished with their windlasses. In real life, line is not self-storing in a standard chain locker.