You might want to reveal the source of this "information. As Firebob noted, if it is just heresay and not based on some previously unknown technical bulletin, etc. then someone is pulling your chain.

There were dealers, Royal and Myrtle Motor most notably, that would perform retrofits of 428s in Firebirds as well as highly tune 400 engines. It is generally believed these cars started out as base 400 models as this would get you better engine cooling, springs, and other components.