Click on "Search" up above and then type in the words "Carburetor Spacers" - it'll get you more than you wanted.....Speedway sells the phenolic spacer sized for the small holes in the early Holley 4150 base / "B" type 57+ manifold. Those original bores fit a 390 (modern) or 400 (original) cfm version of the Holley 4150/4160.
If you use a carb with bigger bores (600 cfm is common) you'll have to "massage" the openings a little to get a smooth match with the holes of the carb tapering down to the manifold size. Otherwise, you'll have to open the manifold up to the carb size - if not there will be a step and other difficulties to deal with.