The 4bbl progressively dumps in the air and fuel compared to the 2bbl that pretty much dumps n the full 500 at one time. Also remember the 500 cfm 2bbl will not flow as much air as a 500 cfm 4bbl because the 2bbl is rated at twice the vaccume leval the 4bbl is. The 500cfm 2bbl is about the same size as 1/2 of a 750 4bbl.

The 2bbl will be a little too much at low speeds and low throttle angles but not flow enough at WOT at higher RPM. Either c be made to work decent but I would still take the small 4bbl.