Chevy 250 & 261,
With a Holley 600, a decent cam, a good head, and headers, I would bet that your volumetric efficiency will be closer to 1.00 than 0.85. In any event, I think that your engine will run fine with the Holley 600. At 6000 rpm and a volumetric efficiency of 0.95, your 250 will want about 412 CFM, about 69% of the rated at flow of the carburetor at 28” of H20 pressure loss. Thus you will see only about 13” of H20 pressure loss, as the pressure loss through a carburetor (or just about any aerodynamic obstruction) varies with the square of the flow rate. At low speed, part-throttle operation, you will be just running off of the primaries, which should be just fine. The only conditions that could be a problem would be low-speed and wide-open-throttle, but with a decent transmission and driving technique you should seldom be in that position.
