I am a lefty too I have an SLR on my 11.5" or my 16" 6.8, and a Syrac on which ever one doesn't have the SLR and both adjustable blocks work great. And yes, an adjustable block is needed when you use a suppressor.
On that note, you will find a significant difference between 6.8 factory loads. For example, the SSA 110gr Barnes ammo is mild stuff, works great on hogs, the Federal/American Eagle FMJ is midlin' and the Hornaday 120 SST is on the hotter side and also works great on hogs. I find I can set a happy medium to handle either the SSA ammo and the American Eagle or the American Eagle and the Hornaday.
JPK