No, all the stock SL-68 barrels are the same length. But because they're a slip fit barrel, you can take almost any barrel for a gun that uses beefy threads (older angel barrels are particularly good) and have them turned down on a lathe to fit the SL-68.
However, you won't see any difference in the flight path of your balls with a longer barrel. They won't fly straighter for longer. Once the ball has left the end of the barrel, it's on its own, the barrel can't affect it.