Tom,
Under boost you are flowing more air than NA so the 600 will be fine, especially on the dyno where your starting pulls most likely over 2000 RPM.
Where a big carb/little motor hurts is NA at part throttle/transient response as there is insufficient signal at the booster to get good atomization and the fuel puddles out causing a bog.
So I suggset you try both the 500 2bbl and the 600. I bet the 600 has better transistion to boost as you have smaller venturis to work with. Also remeber 2bbl's Flows are rated at a different intake vacumm.
My $0.02.
