One cool trick about doing that, if you're careful about total CFM and the static pressure of the plenum, you can remove almost all of the fans that are typically used.  In my Avalon 6 build, I actually unplugged the 120mm fans on them (they were still mounted, as I needed them in place to effectively reduce the intake size) and I just used the extraction fans to pull the air through them.  Nice because my biggest failure point on the BTC miners was usually fans - and when I built the exhaust I used two large extraction fans each behind their own auto-close louvers, so if one failed it wouldn't short-circuit the airflow (although the rig as a whole would certainly run hotter).