We've seen several configurations of the S45, so what we can do and what you need will depend on what you have.
We have an extender (2") if you have an M18x1 threaded barrel that doesn't quite clear the air tank. Two inches is the longest extender that we feel is safe for use on this airgun.
Once you have the barrel threading out past the air tank we recommend going with, at least, the Emperor in .45/50.
You could also go with the Great Kami, which will be quieter than the Emperor.