A larger Rochester 2-barrel (re-calibrated specifically to your engine) would give somewhat better (measureable) performance, with no loss of fuel economy under normal driving conditions.

Most, when they say "500 CFM 2-bbl" are referring to the Holley 4412. This carb would give somewhat better (measurable) performance, with a large loss in fuel economy. In other words, roughly the same performance gain as a larger Pontiac Rochester 2-bbl, without the fuel economy. This unit would not be legal on your vehicle if licensed in the USA.

A Pontiac Carter AFB from the mid 1960's on a 1965 or 1966 intake would give you better performance with about the same or somewhat better fuel economy, but is not legal for your vehicle if it is licensed in the USA.

A properly calibrated Rochester Q-Jet on a 1968~1972 Pontiac manifold (I agree with Vikki that a 1967 would also fit, but it has issues due to the heat passage and is better left to the restorers), would provide much better performance and better fuel economy under normal driving conditions.

A properly calibrated Carter TQ on a 1968-1972 Pontiac manifold (see above) would give you the best performance and better fuel economy under normal driving conditions. I am using this carburetor (part number 9800) with an aluminum 455 H.O. intake on my own high performance Pontiac 350.

Jon.