I don't think the manifold is an issue. Certainly the manifold is not causing the rich run condition.
If the oem says the sensor is to close, then you'll need to follow their suggestion(s).
But you can also do things like verify the timing and the vacuum in the intake. Also verify the plug wires and they're location, fuel pressure?? any of those basics will influence the motor.

Did the oem say how far back to install the bung for the O2 sensor?