Yes, the spreaders pin in the swiveling fitting attached to the mast. From the factory, they are "pinned" with bolts. I use clevis pins like on the shrouds (but longer).
To avoid a spreader end rubbing on the deck, I had been tying up the spreaders to the lifelines - a PITA. As shipped from the factory, the spreader ends were wrapped in carpet scraps.
As far as shroud tension preventing inserting the spreader bars into the swiveling "socket", I have not experienced this issue (but it is close).
To get more information, search the forum for "spreaders"

