One carburetor is always better than two. 500 CFM is far too big to run properly on a 250 engine, however. At 6000 RPM with 100% volumetric efficiency (impossible) the 250 would only be moving 435 CFM. At 90% efficiency, the CFM at 6K would be 398. I'd suggest using a Holley 7448 2 barrel with a 350 CFM rating, adapted to an Offenhauser 4 BBL intake.

We ran the 7448 carb on a bunch of 350 Chevy claimer engines on round tracks, and they would allow the 350 to run 6K RPM, just not at the rated manifold vacuum for the CFM rating of the carb. There are plenty of tuning parts for Holley carbs- - - -main metering jets and power valves, to custom tune the carb for your specific engine and driving habits.
Jerry

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