I see this in a couple of ways.

1. The map was not correct to start with before the build as it shouldn't run that rich.
2. There is an issue with sensor or timing etc that's causing the bad CO% could be many reasons from a dead sensor to the cam timing etc etc.

Did the engine get mapped before the rebuild or at least a decent tune. If it did then I would say that you should take it to a tuner with the spec as is and get it looked over sooner rather than later.

If it you cant get to a tuner in your position id rather run it in using a standard ecu and parts like injectors and map sensor. At least then the fuel should be controlled a bit better. But if there is an issue it will still over fuel.

If was on a good map to start with the fuel side of things would be fine for running in, just sounds like yours for whatever reason is not right.

Overall get it looked at it will only help.