[ericnova72]

Keep in mind a chevy BB is 4 inches longer than a pontiac motor
Best check to see if it will fit
Likely it will, but it will be tight

Under the skin, a '69 Firebird is the same chassis as a '69 Camaro. Factory fitted some of those with 396's....so the BBC into the '69 Firebird is really easy. Factory resto parts exist to make it a bolt-in swap.

The Pontiac peoples are a bit touchy about swapping other makes of engines into muscle era Pontiac cars,
Sso possibly pick up the nearby rebuilt 400Pontiac in the 1980 'bird deal, sell off the Muncie and the SB400 stuff and the 1980 remnants, keep the 454 for '57 chevy.

Or keep the 454 for the '57, sell off everything else to fund an LS swap....the Ls swap is the hot ticket for a nice driver.