I had a similiar problem when running my edelbrock 500 cfm on my Clifford intake on my 250. I bought a metering rod kit and springs and tried all combinations of rods and springs and jets, but still the motor would bog badly when I put my foot in it. The solution was to replace the Clifford with a heated Offy. The throttle response is much better and so is the gas milage. I also went to a one stage leaner rod. I agree with Diesel Dan in that the Clifford had a much better sound, but the Offy performs better on the street.
Al