T/A and Formula are separate models. Formula did not have a "shaker", only T/A (unless you include that abomination that was on "Chips" so long ago...).
BBC is a good engine, no doubt. In a Firebird, though, it will hurt the "balance" of the car drastically (200 additional pounds "up front"). The 455 is more capable for a "driver" in the heavy street car. 440 Pontiac would the HOT lick, but that's another story...