* Factory conversions for specific export markets: Some countries required right-hand drive, but the conversion process wasn't always done by Chevrolet itself, sometimes by local assemblers or dealerships. Tracking these is very difficult.
* Post-production conversions: Many right-hand drive Impalas you see today were converted after they were originally manufactured. These are not included in original factory production figures.
Therefore, a precise answer to your question is unfortunately impossible to give.