My girlfriend is getting a 1969 Pontiac Grand Prix with a 455 in it swapped from a 1971 Grand Prix and Im looking at putting in headers with at least 2/3/4 piping on it. And I would like to know what kind of Horsepower I will aquire from it. Its running a 14.8 right now and it has stock exhuast manifold with 2 piping to cherry bombs. Someone help me out. Thanks.