Just cut away IMO.
If you get problems just crimp the pipes or block one off.
In my experience (and thats mainly with BMC ((I consider SL's to be a superiour)) Pitts mufflers) many engines tends to run better with one stinger totally blocked.
What I've done in the past (because I've used them on many different applications over the years) is to cut them down to say 2cm in lenght and then use silcon tube cut to lenght to suite.