Same engines, the only difference being the Firebird exhaust manifolds flowed a little better than the GTO's due to the chassis configuration.
The biggest difference was in the Q-Jets. The Firebird 400's had a special baseplate that only allowed 3/4 throttle. Official word from Pontiac was that the special carb baseplate was a 'quick fix' by Pontiac engineers to keep their 350 hp GTO engine rated at 330 hp in the Firebird, and keeping it under their 10 lbs. per 1 hp rule. This knocked the Firebird 400 down to 330 hp (in a 3300 lb car), while the GTO was rated at 350 hp (3500 lb car).
A simple adjustment with a pair of pliers on the Firebird Q-jet allowed full throttle, and all the hp came back.
Unofficially, engineers said it was a marketing decision. They knew sales of the new 1967 Firebird 400's would dig heavily into GTO sales, and the sales guys figured it wouldn't be wise to have a car sitting on the same showroom floor as the GTO that could outrun the GTO. But buyers soon found out that in about 30 seconds, and a pair of pliers, they could get full throttle, and the Firebird 400's were always 2/10's faster than GTO's in the 1/4 mile due to their 250 pound weight advantage.
One of the reasons I love Firebird 400's so much. They were the only car that was intentionally slowed down by the factory to prevent it from being "too fast".
