I have Billows that I use with a Smoke X, so not exactly the same set up, but very similar.
An observation though about the Billows itself.
I have used the Billows on a 18" stock WSM, and I found it to provide more air than necessary, causing the WSM to overshoot the target temperature.
This issue may be due to the lack of an adjustable damper on the stock Billows.
Even if the fan isn't running, a significant amount of air seems to still get though the Billows, particularly if the fire is already established and is already drawing by natural convection.
For this reason, if you do get a Billows, I would encourage you to spend the extra few dollars to get the
adjustable damper that permanently replaces the
intake plug.
The damper allows one to throttle the intake to control the amount of air drawn in, whether the fan is running or not.
As always, YMMV, mine is only a sample size of one.