nice photo!
If you have T-tracks on both side decks, they take 1" or 1 1/4" slides that you can find at most marine outlets that sell sailing items (West Marine, for example). Some of these slides have built in blocks and are called geona cars - or you can make up your own from the component parts. If you don't have T-tracks, but do have a perforated toe-rail, you can attach a snatch block on each side - this can then be moved forward or aft as needed, but is more of a hassle than slides on tracks.
