The 428 was not a factory available option for a Firebird. Perhaps there is some crazy story involved with it but for the most part those have been debunked.
1968 400 Coupe, verdoro green, black vinyl top 1968 400 Convertible, verdoro green, black top 1971 Trans Am, cameo white, auto 1970 Buick Skylark Custom Convertible 350-4(driver)