Just an observation. You could potentially shorten the bypass dramatically by making it only long enough to go from the airbox to just after the MAF. By shortening the length, I believe you would reduce the low-speed vs. high-speed airflow difference, and perhaps help keep the top end AFR closer to 13.0.
Instead of tapping the plenum, just tap the elbow right after the MAF (see picture). Just make sure things are as smooth as possible on the inside of that elbow, or you could be doing more harm than good!
BTW: This is some nice creative, old school, engineering.
