Nice progress! We have been away for awhile and I missed you question about how stack location. My first couple builds (done in the dark days of black iron and fire.....long before answers magically appear on the internet) were straight forward stack to cc designs. While they were very ascetic looking, both had drafting issues which took numerous reworks to resolve. Once I started building plenums, draft issues related to exhaust were eliminated. But even then, it took quite a bit of exhaust rework to finally get the cc temps evened out across the racks. Long story to simply say that I have found the sweet spot for the bottom of the plenum seems to be about 1-2 inches above (and centered on) the bottom rack.