.338 Hush in AR10 platform

[SoTex]

Hi everybody, first post here on quarterbore. Glad I found you guys, and thanks for all of the good info I have picked up from lurking around here.
I've found out most of what I need to know to complete my project from hanging around here, but there are a few questions left on my mind. I have all the supplies in place to get my .338 Hush off the ground in my AR10 but I haven't quite got the gas system figured out yet. My goal is to be able to shoot the 180 ballistic silvertip at supersonic velocities and the 300smk at subsonic velocity out of the same rig (1/8 twist). What is the general consensus, can I do this on a fixed gas port system or am I going to have to use an adjustable gas block? Also, is 16.5 the optimal barrel length?