Stay right there and with this diagnostic. Warpage. An intake that did not clamp down properly over a cruddy dowel pin. A bolt hole that is crappy and did not allow the bolt to properly seat all the way in to clench the gasket when it reached its torque. A gasket that is messed up or not the correct one..... something.... You just found your issue. Focus on this one thing. Stay the course. Post pics if you need to.Turned the gas on and stuck it right where the front two intake runners meet the head. And when I did, the miss went away. I checked it a couple times and each time the miss went away and came back as soon as I took it away.
Yes, two lean cylinders can cause two very rich ones in return. The O2 sensor will "balance or average" the total fuel mixture in the exhaust. Two very lean and two very rich will make the O2 seem happy.