Carbs only wants almost as much as Jegs or Summitt for a rebuilt one. The Holley website is around the same. Carbsonly said that 500 CFM is too high for the Slant 6.
That 500cfm is at 3"Hg depression, ....which is not a performance intake density, at 1.5" that is really only a 353cfm carb,...which is not too much for a 225,...especially if it is built.