I, too, suspect
oil, rather than a rich cylinder. I'm not sure how an O2 sensor would would react to burning
oil, but I expect it would report a richer burn.
A good carburetor, no matter how screwed up the intake runners, would get the same AFR to all cylinders. If one barrel was richer than the other, on a dual plane intake, then half the cylinders would be richer. It wouldn't explain two cylinders being rich.
The picture of the heads looks like
oil carbon to me.
You want to make sure that the angle on the intake is correct. Also if the heads have been milled or the block was decked, the intake must be milled to match. In other words the intake may be wider than where the heads are setting.
Set the heads on the block with the old gaskets and torque them down then set the intake on with a gasket on one side and snug the bolts a little on the side with the gasket. Then measure the gap between the head and intake where the gasket is missing. Measure top of port to bottom of port on both ends. See if you are getting the same gap at all four places. Then move the gasket and bolts to the other side and repeat the gap check.
Check to make certain that the intake bolts are not bottoming out before they pull the intake tight. With the intake off screw the intake bolts (all of them) in by hand until they stop. Measure the distance from the head to the bolt head. Then measure the flange of the intake. I had this very issue, on a set of heads. In my case, I had the bolts torqued, and I could slide a 0.003" fealer gauge between the intake flange and the bolt head. Cylinders 1, 4, 5, & 8 were sucking in oil and the four end bolts were not pulling the intake down at all.