If he has a paper trail it is possible, here's why and only why.

First off NO Firebirds came from the factory with 428's PERIOD.

Second - a very few select dealers would put one in for you, one I know for sure did this was Myrtle Motors in New York. I believe Royal did too.

They would have the original paperwork from the dealer and the paperwork better look real old, no copies, it's the only way to prove it. PHS will have no history of this being done but if the papers exist it would be a rare and valuable bird.

A lot of people did this at home too, the 428 is a plug in.